Come on Marrec, this is bad advice, he knows she has a boyfriend and you're telling him to hit it anyway? I mean, okay, they could have an open relationship, but if that's true she should have no problem introducing the two guys. I know if i helped someone cheat i'd feel awful.
Oh ya for sure be careful and don't be a dick and help her cheat on someone but I'm just saying that the term 'boyfriend' doesn't really mean she's involved with anyone.
Here's what you do:
Girl Who Confuses You:
Dralla! You should totally come get some coffee with me later!
Dralla:
But wouldn't your boyfriend be pissed when I seduce you away from him with my amazing charm and good looks?
....
Depending on her answer, go from there.