You’ve never been dumped OP!
I’ve been both dumper and dumpee on a roughly equal number of occasions. She’ll be in shock, and grief for what might have been, her sense of self worth will be hurting, she’ll be asking why did he say he would then didn’t, what was that all about, what’s not good enough about me? I can honestly say -NOTHING. It’ll have more to do with whatever motivates him than what’s ‘wrong’ with her.
We can deduce he didn’t fancy living here, the exciting travel part was over. It probably all got a bit real, and IME when you dump someone it’s surprisingly seldom to do with there being anything amiss with the person themselves (all my dumped boyfriends were very nice men who have gone on to meet women who I would say are objectively ‘better’ than me, by which I mean, more attractive, fitter, stellar careers). Strangely, the ones who dumped me, not so much. In fact I’d go so far as to say they’d have been better off with marvellous me but obviously that wasn’t how they felt at all so hey ho! Also, on two occasions which really broke my heart, I had to confess (much later) they were probably right, and it wouldn’t have gone the distance.
That might be my (bitter and deluded) perception, of course, but it’s a POV it may help to share with your daughter.
Relationships are mysterious, people don’t have a P/L account of attributes and faults running in their head and our decisions about who to be with are usually based primarily on feelings, which we all know we can’t really control. I have known at least one excellent man that just didn’t do it for me. On paper it should have been a match made in heaven and I see how his life is now and think argh why couldn’t I just have wanted him 😂
On that note, also totally possible that some people don’t know a good thing when they’ve got it, so she REALLY shouldn’t draw any negative conclusions about herself. He did her a solid by bailing when he did, she wouldn’t want to be stuck in a lease - and a life - with someone who didn’t want to be there with every atom of his being.