I think what he meant was, go meet with her once, tell her that you already have a gf and aren't interested, and don't pull anything funny.
Then again, I don't know your gf, is she the type that this could be a trick? Then you might wanna just say "no" without arranging a meeting as you never know when she could jump out and say "ha! I caught you cheating" before you can tell her friend to take a hike. Especially since she somehow mysteriously got your e-mail, this could be the case. Your gf could be testing your honesty, and wants you to tell her.
Finally, the last option is just to pretend you never got the e-mail.