Chaos_ult: I wouldn't ever date/hookup with anyone who is in a relationship. Even though you are not at fault, I am one of those moral idiots
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I think that if you are very serious about this girl you could try to date her and start a serious relationship with her. If you just wanna get laid, pick one of the other fish in the sea that is not taken.
If she wouldn't break up with her bf right away when you start dating you should probably gtfo for a relationship since she obviously cannot be trusted.