I have Classical PKU. I've been off the diet for 32 years! I just went back on a modified diet on 11/30/11, along with drug therapy started in September 2008.
She should be fine, but do make sure she sticks to her diet. I was taken off when I was 7. I know she's young, but had I not had that burger or meatloaf, I would've never missed it. Converting back was not as difficult as it sounds, if you keep the frame of mind that it's needed for a healthy life.
Also, had she been sick? I know from experience that a cold, flu, or infection could cause a spike in Phe levels. It's happened to me many times.
You can add me to your friend list. If you have questions at any time, I'd be glad to help.