When I was younger, I used to try and reassure the GF she has nothing to worry about etc., going out with friends would be really stressful and I'd leave my imaginary balls at home, or cave in and wouldn't go out. But now, I don't have patience for insecure girls who can't handle a relationship. I suggest he talks to her and reassure her once or max twice, but he mustn't change the way he acts because of her problem. In the end it's her problem, not his. If it doesn't work soft next her until she behaves normally, otherwise move on. Hope this helps!