1) Never hook up with them
2) Only date them.
Our dating experts discuss whether it’s ever okay to date your BFF’s ex:
(Jozen Cumming) HE SAID:
Dating a friend’s ex takes guts, that’s why I always advise people who do it to make sure they’re dating the friend’s ex for all the right reasons. I would never encourage sneaking around behind a friend’s back with their ex, but if you and the friend’s ex are not really going to be in a serious relationship and only want to have some fun with each other, keep that to yourself until the fun wears out. But if you two decide the feelings are serious to the point where you two want to do things like walk in public together holding hands, then it’s time to face your friend and tell them exactly what the deal is.
The question is not is this right or is this wrong, because that isn’t for you to decide. What you have to decide is how much you’re willing to risk upsetting your friend for the sake of being with someone you feel so strongly about and for that relationship to be misunderstood. No matter how much you say you care about the person who was once your friend’s significant other, your friend is going to see it as an attack.
I always say, if you’re going to date a friend’s ex, you have to be able to look your friend in the eye, tell them you’re now the new person in their ex’s life, then be willing to get punched in the eye. If you’re not willing to accept your fate, you probably don’t like your friend’s ex as much as you think you do.