Young people learn about relationships, about themselves, and about what they want in a relationship by BEING in relationships. I'd imagine this young man learned quite a lot in his last one, and hopefully, he is a better, more mature man for it.
I think it takes a great deal of maturity, insight, and courage in instances where a young person breaks off an engagement. By the time a couple is engaged, there is so much time and emotion invested in the relationship, and possibly wedding plans, that many young people talk themselves out of their own truth and go through with the marriage only to go through a divorce later. This is not only hurtful to them and their families, but also to any children involved.
I broke off an engagement when I was in my mid-20s. He was a great guy, came from a great family. Our families loved each other, and we had been together for five plus years.
In my heart, though, I knew it wasn't right. He deserved to be loved completely, and I knew I could not give him that. I had to do the right thing, and it was the hardest thing I'd ever had to do.
Flash forward a few years later. He met a great woman. They were married and have a beautiful family. They've been married almost 30 years.
You need to trust your daughter's judgement and let her make the decisions about her relationships without interference. If she wants to talk to you about it, be there for her, of course, but she is the one who has to make her own decisions here.
Let your daughter be. Let her learn about what she wants in a relationship, and let her find it on her terms.