If this guy has a girlfriend already but asks you out, then this guy is a jerk. Don't accept his offer. Do you really want to go out with someone who would cheat on his girlfriend? Plus you're friends with his girlfriend and in the long run, friends are better since you are less likely to lose friendships than you are with a romantic relationship. Also think about it this way. If you were in your friend's position and your boyfriend asks your friend out, would you like it if your friend said yes? Probably not. So don't say yes to this guy and avoid him. He is not a good guy. You deserve so much better!
If you really feel like you're in a caring mood, then you should not only tell this guy to get lost, but you should also tell your friend about this. Tell her how her boyfriend is a jerk and that she should just dump him. Then both of you can avoid this guy together.