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.

Does the girl know they in a relationship?

Or is he fantasying about a relationship that does not actually exist and then getting upset when reality intrudes on that fantasy?

Meditation comes in various forms, one of which that could be useful is focused attention meditation.

Basically you focus all your attention on one thing and do everything you can to keep your attention on that thing until the timer goes off.

An example could be like a dime. You out a dime on the table and look at it. And you try to focus all your attention on it. Your attention goes elsewhere, but you keep bringing it back.

It’s very difficult but it can be improved.

What I found is that after I did that kind of meditation for 5 minutes a day, for a few weeks, it was easier to (a) notice when I was thinking about something specific, and (b) just stop thinking about it, and refocus on something else.

In other words it improved my metacognition and my attentional control.

Those are two attributes that can empower a person to better handle and avoid obsession or rumination problems.