This belongs in the category of high drama, if you decide to get involved. Keep your sanity and let them do their thing. This sounds like a soap opera in the making, these are always viewed best from...
you can tell if he looks at you in crowds or he says "he knows who likes you" something like that then yes. It also may depend on who broke up with who.
The greatest gift that you could give to yourself is respect. You dont have to go after the guy/girl if he/she already leaves you. If you truly love him/her, you can accept him/her when he/she goes...
If you hate his ex try reading I HATE HIS EX by Alex Cooper. I had loads of problems with my fiance's ex and I have now resolved them thanks to the advice in this book! You can get it on Amazon!...