Were you being nice to her for the sole purpose of being nice, or were you being nice so you can get ass in return?
I mean, if you were just doing it to be nice to her, and you had no romantic interest in her, then what would it matter if she slept with one of your friends (granted, its shitty he was married)?
Now if you were being nice to her because you just wanted ass, than that's not right either.
Just trying to get philosophical.
