The rules: dating your friend's ex-boyfriend

Question his motives

Even mind-blowing sex ultimately isn’t worth sacrificing a friendship, so make sure you and Mr. Ex both see the relationship heading in the same direction and that his motivations are on the up and up. After all, it’s one thing if he was your friend’s college flame and you bump into him 10 years later in an Italian cooking class and bond over lasagna. It’s quite another if he dumped her last weekend and wants to hook up with you now.

'It’s a red flag if a guy wants to date you right after he broke up with your best friend, especially if they were serious,' warns Burton. 'If he’s flirting with you a few weeks later, it’s probably more to make her upset and show he can get a girl who’s supposed to be off-limits, than any genuine interest in you.'

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Take it slow!

If your conscience gives you the green light to pursue Mr. Ex, still take things slow, even more slowly than you normally might with any other guy. Though the temptation will be incredibly strong, 'don’t jump right into bed with this guy,' counsels dating expert Hayley Quinn. 'Build your relationship in a more public way, with daytime activities, coffee dates, lunches. That’s safer because you’re less likely to get carried away and end up in bed.

Going slowly also gives you time to exit if it looks like it’s not going anywhere.' (And, Quinn notes, if the relationship is a non-starter, you can easily explain away a few coffees and lunches, and no one needs to know a thing about it).

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‘fess up to your friend fast

If you may have something special with Mr. Ex, your friend needs to know. And she needs to hear the news from you. Dating experts differ on when's best to speak up. Burton thinks anything less than full-disclosure on first contact could be seen as 'sneaky'.

Others say wait unit there’s something to report. 'Once you’ve had that first kiss and feel like your relationship could be more than casual, then you need to tell, and certainly if you’ve already been to bed,' says relationship and dating expert Julie Spira, founder of Cyberdatingexpert.com.

Since there’s no universe in which this will be a pleasant, or even easy, conversation to have, keep it short, simple and honest: I don’t want to hurt your feelings, but I don’t want to hide this from you either. I’ve been going out with Mr. Ex.

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Prepare for the backlash

If telling your friend you’re dating her ex was hard, this next part may be harder. Once everything’s out in the open, you’ve got to sit still with your mouth shut and brace yourself for the emotional backlash.

'Expect a very angry, jealous response,' predicts Yahoo’s mental health expert Rob Dobrenski, Ph.D., author of Crazy: Notes On and Off the Couch.'Don’t go in with the idea of defending what you did because that says to your friend ‘You’re not allowed to feel what you’re feeling right now.’ But if you let your friend have her say, she may come around at some point.'

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Follow your friend’s lead

As the wounded party in this situation, your pal (if she remains one) gets to set the tone for how friendly she wants to still be with you… and how much she wants to see and hear about you as a couple. 'If you’re breaking this taboo, you’re going to have to acquiesce to your friend’s comfort level to try to salvage your friendship,' says Paz.

That said, the best way to show that you still want to be a friend is to spend time with her one-on-one and continue to include her in group gatherings even if her ex, your new beau, will be there, too. 'She can always say she’s busy if she’s uncomfortable, but at least you’ve done the right thing by inviting her,' says Spira. 'I see too many women dumping friends for men and then if that relationship ends, they’ve lost a friend.'

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Be prepared to lose your friend

If you date a friend’s ex, be prepared for her to wash her hands of you. 'You’re going to feel sad, but you’re just going to have to deal with it,' explains Dobrenski. Still, it’s possible that after some time has passed, you may be able to heal the rift.

'I’ve had clients who’ve reconnected with friends, sometimes weeks, sometimes years, after the dust has settled.' But you’re going to have to be patient. 'Don’t push it. That’s doubly unfair to your friend who’s lost both her ex and you.'

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Don’t spread the news on Facebook

Naturally, you want to tell everyone your new relationship status, but resist doing it on Facebook, cautions Dobrenski. Assuming you’re still Facebook friends with your pal, you should be especially sensitive about posting updates and photos that could potentially hurt her feelings.

'You don’t know who will see that and it could lead to some nasty postings on your wall,' he says. Instead, personally tell a few close friends and then let the grapevine do its work. 'Depending on how your friend reacted to the news, I would say no posting pictures of the two of you on Facebook for six months to a year,' says Paz. 'Take the high road where your friend is concerned rather than trying to assert your new position as Girlfriend.'

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Ease into your social life as a couple

When you and your girlfriend move in the same social circle, presenting her ex-boyfriend as your new man requires some delicate maneuvering. 'The crowd reaction can be a lot more hostile because a pack mentality takes over,' explains Quinn.

'If you have one outspoken friend who’s vehemently opposed to your new relationship, she can affect how everyone else in the group responds, even if they might have been fine with it.' You may get a better (less judgmental) reception if you plan small get-togethers with just a few friends at a time.

'Do it at your place, on your turf, and it won’t feel like you’re imposing your new status on the group,' says Paz. Don’t push and the crowd will likely welcome you back in time. But if you continue to feel frozen out, spend time with friends who support you and seek out some new friends who don’t know your backstory.

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Don’t share relationship details with her

You may have shared the minutiae of your past relationships with your gal pal, from the weird way the last guy you dated chewed his food to that thing he did with his hips that drove you wild. But when it comes to her ex, silence on matters both sexual and not really is golden.

'Even in the best case scenario where you get your friend’s blessing, tread really lightly on how much you share,' advises Paz. 'Even if we’re over someone, we can still be a little territorial and competitive, so something as minor as you getting along well with his mother if she never did can really sting.'

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