[fliptuner]

1. You're calling him your bro and think you'll feel guilty. Dont' you think there's a reason for that? If he's your "bro", you wouldn't fuck with his ex without talking to him about it first.

2+3. Your friendship w/ him or her? If he's cool w/ it, fine. If he's not, you're gonna lose him as a friend. Her? IMO, sounds like she wants to use you as her fuck toy til she leaves and you're already friendzoned. It also seems like it's easier for you to mess w/ her cause she's already there and willing to put out. He probably only tells you what he thinks you're going to tell her anyways since he knows you two are close. She's leaving anyways, so what are you going to do, start a relationship with this chick that's leaving, barely have a relationship with and be stuck here, w/ your ex-friend, who's ex you were screwing w/, right after they broke up?

4. No.

You sound young, there's plenty of girls out there that aren't your friend's ex, go find one of them.