Lots of people have told me unequivocally that they would never date a friend’s ex. They wholeheartedly believe that it’s wrong, disrespectful, and if a friend did that to them, they’d never talk to that person again. Dating your friend’s ex IS possible and can be very successful too, you don’t have to lose either your friend or your lover in the process. Here are a couple of rules to help you out if you ever do consider dating your friend’s ex.
1. Don’t trash talk. It’s OK to come to your partner for advice if you’re arguing with your friend, or vice versa, but absolutely resist the urge to belittle or insult one of them to the other. This can be extremely tempting if they ended on bad terms and you know you’ll find a sympathetic ear. However, in order to maintain a healthy relationship with both of them, it’s crucial that you never seem even a little like you’re taking sides in their breakup or casting either one as the bad guy, even months or years after the fact. If you need to vent about one of them, find a neutral party.
2. Respect boundaries without making assumptions.For instance, if your friend doesn’t want to go to parties where her ex will be in attendance, don’t pressure her. But don’t assume she doesn’t want an invite if you haven’t asked! In general, allow your friend and your sweetheart to decide how much contact they want with each other, and don’t push them to associate if they’re not into it. Remember that you can love them both without them necessarily having to enjoy each other. This goes for friends and partners who haven’t dated, too, now that I think of it. Set aside time for each of them and honor it — don’t drag your lover along on girls’ night out (not even if your lover is a lady; queer chicks are so bad about this), and don’t invite your friend to what was supposed to be a romantic dinner at home.
3. No comparisons. Don’t ask your man if you’re prettier/smarter/better at Scrabble than his last girlfriend.Don’t do this ever, but especially not if his last girlfriend is the person you’re going rock climbing with Sunday. No matter what his answer is, it’s going to make things weird. Besides, comparing yourself to anybody — even if you come out ahead — is always going to lead to feeling crappy, because basing your self-esteem on where you stand relative to someone else is Not Healthy. So don’t seek out comparisons, and if your dude brings up the topic, tell him you’re not interested in hearing it. You and your friend are not in competition, except when you’re actually playing Scrabble.
4. Don’t be paranoid.Don’t try to keep your partner and your friend from associating because you’re afraid they still have feelings for each other, and don’t constantly seek reassurance that that’s not the case. Trust that your dude is with you because he likes you and you’re awesome, not because he’s biding his time until your friend takes him back. Trust that your friend is happy you’ve found someone you dig, not plotting to sabotage your love. And don’t ever use jealousy or insecurity over their past relationship to excuse irrational or controlling behavior on your part. Of course, if your partner gives you a legitimate reason to believe he’s untrustworthy, get out of there stat, but if there’s really nothing wrong, don’t create problems where none exist.
5. Don’t pry into their relationship.It may be tempting ask your friend to analyze what happened between the two of them so that you can avoid making the same mistakes, but resist that urge. Likewise, don’t grill your boyfriend on what went wrong or insist that he account for his behavior throughout the entire time they dated. Their relationship is between them; it’s not your cautionary tale or your soap opera. If they choose to share details with you, that’s fine — you don’t need to stick your fingers in your ears, unless an overt comparison is being made (see No. 4) — but don’t push. Your relationship and theirs are separate things, and you don’t need to know anything they don’t care to tell you.
6. Recognize that some exes really are off-limits.It’s easier, of course, to have hard-line rules — “exes are never OK” versus “exes are totally fine” — but that’s not the world we live in. If someone seriously mistreated your friend (we’re talking emotional or physical abuse, infidelity, lying, stealing, etc.), don’t date him, no matter how awesome his butt looks in jeans. This has nothing to do with some kind of Eternal Dibs situation, and everything to do with the fact that, by choosing to build a relationship with someone who treated her horribly, you’re telling your friend you don’t think what he did to her was all that bad. Just walk away. There are lots of people out there who are just as good in bed and haven’t traumatized anyone you care about. Set the precedent that people who are awful to your friends are people who don’t get to see you n***d, and your life will be the better.

Low down dirty
Are you trying to get back at your ex
Dumb thing to do