It may help your daughter to understand what abuse is, why it's used and that abuse issues are not easily solved. The young man may have good intentions, but it usually takes more than an apology and a promise to change, to make real changes. She needs to know that she can't change anyone else, they have to do that. She doesn't have to go into the shelter to get some information about abuse. Even one hour spent with someone who deals with this daily can be a real eye-opener.
Abusers start out as teenagers.
They usually get worse the older they get. Relationships with abusers can seem wonderful at the beginning, and after a fight. This is part of the cycle which repeats over and over. It can also be helpful for her to learn to recognize the danger signs of an abuser. Hang in there, Mom.