Ok. The question is, did he promise the old girl marriage?. If yes, i wouldn't advise the guy to leave. Him moving on to another usually doesn't end well
But rather he should sit her down and talk about her attitudes and behaviors, because that seems like the only reason he left. And since he once loved her, they should find a way to rekindle their old love instead of him moving on to another.
Now you come. Thank you so much. This is in line with what I told him.