what i would be asking myself is why my girl wants to bed another girl? either way no good can come of it, one night of "fun" could destroy what you have going your girl, but funny you mentioned you didnt know her, what if you did know her would that of made it easier to choose?
maybe im old fashion but when i love someone i don't like to share her with someone else, heres a question to ponder what if she wanted another guy in that 3some?