I don’t disagree with what others are saying but I want to add another perspective. Speaking from personal experience, I sought the approval of many girlfriends because I didn’t get approval from my mother. As cliché as it was, after making that realization it became more apparent that I had very little self worth.

If your friend is anything like me in this scenario, it might be better to not focus on the girlfriend situation and focus more on him. What is good at? What makes him unique? And above all, give the reassurance that, “Your worth is not dependent on other people’s opinions.” If I had someone tell me that earlier, it might have saved me some grief. But people cannot forced to be changed, so you can give that advice, and he might continue doing what he is doing. But hopefully it plants the seed for later introspection.