Dating exs child
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.They believe this is something everybody knows, that they're just following the rules.
(I'm going to use female pronouns for your friend, and male pronouns for your sweetie, for the sake of simplicity; however, every rule here applies no matter the genders of the participants.) Keep your friend's secrets.
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. 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.
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.
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.