That girl left your friend. Yes. she might have to do it since her mother was opposed to the union. But at the end of the day, she left him. Your friend was not good enough for her to stick with him. His love was not enough. Your friend was not her choice. So, she (had to) left.
These are the things in your friend's head now. Why should he marry someone who left him when the goings were rough. That's like a slap. Who's to say, she won't do it again? We men need women who are loyal, affectionate to us. And we don't like to become someone's second choice. I'm pretty sure your friends might be having these thoughts.
Btw, what's that girl's mother saying now?