That's a sticky situation. On one hand, he's an adult now. On the other, you have a lot invested in him.
It depends on how she responds to treatment, I guess. If she does well she can lead a somewhat normal life and they can have a good relationship. But, yeah, I guess it could go wrong. I've dated my share of these types of girls when I was his age.
From what I know about
SSRI treatment (which isn't much....I'm no doctor), the meds will actually increase anxiety initially. Hopefully some people will come on here to confirm that with their first-hand experience. It may explain why she's actually gotten worse lately. Hopefully she'll level off soon and then get better.
Point her and her parents to sites like this one. People DO get better. I never thought I would but I did, and if I did she can too.
My Brain: My friend, My enemy: A blog to chronicle my attempt to recover from anxiety/panic disorderanxietypanicdisorder.blogspot.com/