Can someone explain this please?

My guy flirted with his mates girl one night.we had all a bit much to drink,but me and my man weren't invited back.his mate took offense.last night I was driving,and his mates girl happened to be behind us alone.I watced my man look in the side mirror,then he stopped.he pulled down his overhead mirror and pretended to be preening himself the whole time she was behind.why was this??