The more he tries to "win her back," the more she'll pull away. Putting this into practice when you're emotionally attached is hard because when you're in that state you think you just have to try harder and put in more effort, but all this does is push her away even more. The more you try to get her interest back, the less interested she becomes.

It's much better to just completely detach and let go. If it's meant to be and she's genuinely interested, she'll come around. If she doesn't come around and has lost interest, then no amount of effort will change that.

I know women will say how he should "communicate" but this is terrible advice. No amount of communication will make a woman who has lost interest in you regain it.