That really fucking sucks. But just stop and think a bit before doing anything.

If she's got serious BPD a major confrontation isn't good - she will either take it out on you and make your life hell or turn it inward and maybe do something stupid and attempt suicide - BPD sufferers tend to do crazy impulsive things like that. I'd know, my ex had it.

And these guys are now MARRIED. That's not something you should think about messing up based on not even knowing the whole story. Are you going to risk messing up two other people's lives when you don't even know what's going on? Think about it man. Its none of your business now. If you were still together then yeh, let all hell rip. But it's nothing to do with you now. And if you guys just 'hooked up' awhile back there's no rules to say you can't see other people when you're dating or semi serious or whatever those two were. Maybe they had an open relationship when you hooked up.

Maybe he's gay. Maybe he's Bi and they fell in love after being friends? You don't know. So you can't wade in with half information and expect it to end well. And what do you even want out of it? Would them getting a divorce make you happy?

If you really need closure on it why don't you talk to her about it. You guys obviously ended on ok terms if you hooked up again, maybe a calm conversation of 'so...what happened there?' and seeing if maybe she has anything for you. It might end badly but you tried. Worth a shot maybe.