OP: I haven’t read all of the posts so I do not know whether or not this has been addressed.
A significant concern with your attachment to your daughter’s ex-boyfriend is that your daughter may feel guilty about breaking up with him because it hurt you.
Your daughter is young & growing. She needs to be able to make unencumbered decisions about her relationships.
Although this seems simple & obvious, it can cause pain, guilt, and confusion to the young adult whose parent is involved in his or her relationship with another.
Moreover, it seems clear that the young man’s mother did not want this relationship to continue. Her stance did & will affect her son.
Young people need time to grow and to experience life. And experience is not always pleasant. But a child should always be able to count on his or her parents being there to support him or her–not to raise feelings of guilt, doubt, and insecurity.