I like my BFF but he's stuck in a relationship he doesn't want but can't seem to end

I think I like him quite a bit we do stuff whenever we see each other practically and then he got back with her and it kind of stopped bc he knows it's wrong to cheat but then last night we did stuff again and idk if he's with her or not.  

I know it's wrong bc I wouldn't want anyone touching my man if I was with someone, the situation is just messed up and he doesn't want to be with her but has a lot of bad stuff going on ATM and doesn't want her to die or self harm etc but idk what to do

He likes me back, we're just in a bad situation and it's not rly my fault but idk how to tell him that I  can't just sit here and wait, I don't know what to do but she is very possessive and he is kinda vulnerable bc of what has recently happened in his life, please give your views he is so kind to me, isn't like a fck boy, is lovely 

Every time he ends it with her they get back together because she cries to him and he can't handle that...