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A: Your friend may not know others think he is flirting. Some people are naturally flirtatious and do not realize how others see their actions. Talk to him, he probably doesn’t know that the girls in your group are becoming irritated with his flirting. Tell him that the girls would like him to cut back on it. Be ready for him to say that he doesn't flirt with them, because he may not know he's doing it. Let him know some specific situations that have made the girls uncomfortable.
Don't expect a complete character overhaul, but if he knows specific things that are irritating them he can stop doing those things. If we do not let someone know the things that are bothering us we cannot hold them responsible for those things. Until it is communicated the other party does not know their actions are bothersome. You cannot force your friend to stop, but letting him know that he is being perceived as irritating is a good way to help him choose to stop.
Alex