If they’re just venting, let them. Most likely they just need to get it off their chest, plus the feeling of being heard/listened to is very validating in a lot of circumstances.

My best friend and I will usually give a heads up (like, is it okay if I vent for a bit? Type thing) and we just, listen. At the end we usually go “that really sucks, I’m sorry it happened to you” or we often agree “yeah I’d be frustrated too, he/she/they sound like they’re a real peach”. If it’s someone I care about venting, I mainly just make sure they’re feeling that they’re being heard, even if I can’t help.

I try not to offer advice or talk about my own experiences unless asked.