If you care about your girl, you wont. If you don't give a shit, ask her straight up. One of three things could happen, you could lose your girl and not tag her friend. Number two, you could lose your girl and fuck her friend or number three...you could keep your girl and hit the friend and all will be right with the world.
Your gf might be testing you. They might be setting you up to see if you will take the bait. Why even have a gf or wife if you going to fuck around on them. One woman is enough to take care of more less two.