I actually wrote like 2 paragraphs about openly talking about the situation with him and being direct without sugar coating it. Which I still believe is the correct decision.

But as a person who's rebounded and has been a rebound. There's nothing wrong with that and the stigma placed on it isn't really true half the time. If you're in a shitty relationship and it ends why wouldn't you want to go next. Some people decide to live single for a bit, I did that after one relationship. Some people already see a failing relationship but aren't driven enough to end it till the other person does, at this point both parties are looking for the next.

She didn't break up with him out of the blue. Dumb arguments etc are signs of a failing relationship. Sure they happen in all relationships, but if you're venting about them rather than joking; I feel like it's on a dead course. If you're worried about being a rebound, you're correct in thinking that. But you've made your move, regardless if you're happy with it or not. You like the guy. Just have an open conversation about how you liked the person he was and not who he is now. Straight up ask why he is like that. Remember that you were his pseudo-therapist and now he can't say things about you to you.

Rebounding doesn't automatically mean a doomed relationship.