He is a jerk? You don’t have to try and figure out why he’s doing this. This is not at all uncommon for army boys. I say that with a long line of admirable men in the forces, but a lot of young men will say whatever they need to to have a gf experience and comfort before they get shipped to the next area, next woman.
She hasn’t known him very long, and has invested way too much of herself with him. How does someone become inseparable if they are in college and their bf isn’t? Where are her friends in this? Does she have them?
You should encourage her to focus on her and her needs and wants and focus more on her life instead of getting so sidetracked by a guy who clearly didn’t share the same feelings. She should also have counseling available at school that would be really good for her.