L.H.
S....I believe you gotta let her go. Even if this is the boyfriend for her, she is WAY too young to commit to one person right now. She needs to go out and experience life, good and bad relationships, before she will ever really know what she wants/needs out of a spouse. We all change SO MUCH between the ages of 18 and 25.
I married my high school sweetheard when I was 20...we were divorced a year and a half later.
Let her go and experience life.
L.