Dating the ex
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 going to lead to feeling crappy, because basing your self-esteem on where you stand relative to someone else is Not Healthy.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.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.5. Don't try to keep your boyfriend and your bud 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.
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.
The reverse is also true; no matter how much you love discussing your dude with your besties, his ex can probably live without hearing the details of his current sex life. 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.
Save it for your diary or for anyone who didn't date him. This can be extremely tempting if they ended on bad terms and you know you'll find a sympathetic ear.
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. Your relationship and theirs are separate things, and you don't need to know anything they don't care to tell you. 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. There are lots of people out there who are just as good in bed and haven't traumatized anyone you care about.