Drywall has no structural use. You need to have a professional at least evaluate it. Is that the only bathroom? You need to at least shut off the water to that toilet if that’s what’s leaking.

Photos please.

And if the drywall is already messed up, yank it off and see what is underneath, and take more photos. We can’t assess the seriousness without seeing under the drywall.

1. Cut out as much of the rotted stuff as you can that isn't structural

2. Identify the source of the leaking and stop any leaks, or if you are unable to handle that yourself, turn off the water to the toilet and drain it until a more qualified person is able to do so. If that is the only toilet in the house you should not be using it anyways and this is definitely an emergency situation that you should reach out to any and everyone possible to get fixed asap.

3. Once the leak is stopped you can asses the damage to the structural portion and remediate. This is where further advice becomes useless without having actually seen the situation

If stuff has been leaking, the floor joists are probably rotting and you'll have to do a full teardown of the wall drywall and floor and replace them. Hopefully the walls are not compromised. This isn't cheap, but this is something that has to be done correctly.

Possibly, though I have seen it not be the case before. Had a leaky bathtub drain that rotted a bit of the ceiling, pulled out the drywall, the floor sheeting around the drain was fine and so were joists, it had been dripping down the u bend just onto the drywall below. Sometimes you get lucky and things aren't as bad as they seem. Not always

1. Shut off the water.
2. Remove the toilet. (Take the tank off, set the toilet on a mover's dolly
3. Repair the plumbing, floor, and the subfloor
4. Reinstall the toilet and test.
5. Repair the ceiling.

As for the cause, a failing wax ring (or closet flange) is the likely cause--just shooting from the hip here as you probably would have noticed the smell of shit water leaking in large quantities

I wouldn't remove the tank. Be careful with the lid though.

I always do if I'm re-seating a toilet. Makes placing the bowl on the new wax ring easier because I have to lift less weight

Likely just superficial drywall crumbling because of moisture. Agree that most common source is the flange. Seems scary but not the hardest job in the world. You can use a screwdriver to test the wood. If you can stab into the wood and it feels more like stabbing into dense bread/cake it’s rotten. Also, if you’re walking around upstairs and it feels bouncy or the floor is squishy, it’s rotten.

If it’s solid, leave it. Otherwise, replace rotten pieces. Subfloor is pretty easy to replace. Talk to a pro if the joists are damaged. Leave water turned off and dry the exposed 2nd floor joists and subfloor with a fan in the kitchen for 24ish hours. Next, once everything is dry, put a couple of tablespoons of bleach into a 32oz spray bottle and fill the rest with water. Mix it up well and spray any areas just enough to coat that we’re exposed to the moisture. This should kill any mold spores that may have set up. If there’s black or slimy stuff in there, throw a good mask on and clean it up before drying. Other commenters had good instructions for re-seating the toilet on a fresh flange. Patch in drywall once all is done. Also not a bad idea to just throw a small frame around the hole with some 1x4 wood if you’re not too worried about aesthetics and leave the center piece in the middle detached for future access. Plan on a weekend of work.

Wear gloves. Just in case it's not water.

not to mention that it's created a biohazard. ew!

"part of her kitchen ceiling, underneath the bathroom, is busted up." Drywall repair is going to be the least expensive part of the repair. I agree with others who have said call a professional. They will take the drywall down and assess what needs to be done. But in the mean time make sure mom covers her spaghetti sauce with a lid.

I like being able to lift it without bending so much.

If you have no idea how to fix and limited resources, you may find a couple of books handy. Do you have a library available to you? You'll need a plumbing and carpentry book. You absolutely have to open the kitchen ceiling and find the leak source! It will also allow you to assess all of the wood structure including the bathroom subfloor and joists. I would hope the joists that hold up the entire floor are not rotten. If they are, I hope your mother has some equity in the home because you will need a professional to fix that level of damage. But if the joists are solid you could pull this off with some books to instruct you and perhaps some friends to help. Pictures would help considerably. Well lit, focused pictures.

I think everyone appreciates your suggestion and I can tell you put a lot of effort into it just want to advise on the cut out what isn't structural part. Its difficult for people without diy experience to tell what is and isn't structural and something like this should probably be stripped of drywall and subflooring then temporarily supported from below while the rot is removed. That can be very overwhelming for a first timer. Just make sure someone around knows what they're doing at least somewhat before you start removing and beams from the floor, even damaged ones.

I suspect OP doesn’t know how to tell what’s structural, at least not at this point.

Any mold inside is a biohazard to some degree

It's probably not a good idea for op to try to diy this from a few sentences off the internet then...

You've been given a lot of great suggestions here but if you have no experience with DIY home repairs and little financial resources, seek out friends and family with skills and beg them to help you. It's a great way to learn.

Dude no! You gotta fix that leak. Rip that drywall down asap and start looking for the leak.

Man if that toilet falls it would be pretty shit 😹

If she has insurance call them!!!

The best advice I can give you is to sister some new temporary joists to the existing. They'll either be 2x10 or 2x12 that you'll need. Do not remove anything existing either,as you could easily compromise the structural integrity that's there now.You'll have to pull down most of the drywall on the ceiling to accommodate the sistered joists,as you'll have to join the temporary joists as far back onto the existing where there is no water damage with an overlap of at least 6 to 8 feet on both ends,or worse case scenario, all the way to your top plates on the outer walls. If the joists are bad enough, you may need to sister on both sides of the existing with cripples in between the joists for more structural integrity until you can properly fix the problem

Below A bathroom. This means standing water has seeped first at the edges down into the wall and then slowly rotting the floor. If the wall below is showing that much rot then you have bigger problems then just the floor/ceiling.

Does your mom rent or own?

Maybe lay down on the taco bell

Time to do some homework and get resourceful. Have someone come look at it just get some information. Might give you a place to start. Also, tap your network. Ask if anyone of your friends knows someone who can help.

Pictures would help. I had a house with well water and it was so cold condensation would form and that caused rot. I’d turn off the water if you have another toilet. Remove the loose and damaged drywall. Take photos then tape up a white tarp or opaque plastic sheet over the hole so it doesn’t look terrible and nothing falls into the kitchen. Post photos. Depending on what is revealed this might be a job you can fix for not a lot of money.

Keen to see some photos, but definitely shut the water off to that toilet and try and isolate the issue first.

What do you mean?!?!? You can't see through drywall????

Mom has kryptonite in the kitchen, my x-ray vision isn't working...

Is the toilet plumbing still leaking? Stopping that is step number 1. Sounds like there is already significant damage and the right course of action here will be to gut the bathroom and pull up the flooring, identify how much rot there is, remove and replace the rot, then install new flooring and reinstall the bathroom fixtures.

If there is no septic smell, could be a crack in the toilet or tank.

Call a professional. That's what you should do.

maybe focus on fixing the toilet first.

NO ONE can give you accurate answers without photos. Even then it might be a little risque.

Stop using that bathroom and start slowly taking everything out of it. First to go should be the tub. Wanna save money, demo and remove everything that you can and then bring in someone (a few estimates are good) to evaluate and go from there.

Yea if you gave us pics I could give you a quick fix till your $ is up

I can, I’m just lazy.