Ooooohhhh, boy do I understand that! I had the exact opposite problem in high school (I'm assuming that's where she is?) -- I got physics just fine but never could "get" biology, and things usually come easily to me as well.
Tell her to hang in there, and try to check out other sources of teaching. If her teacher and that textbook aren't helping, if she really wants to "get it", check out *different* textbooks. I had to learn how to do that in college and I discovered that often times one book would teach in such a way that I never could understand it, but another different book would finally make things click.
She should also try some hands-on things -- if she observes what's going on, then tries to figure it out, it might help rather than just looking at problems mathematically. Assuming what she's working on is something she can do that with -- kinda hard to observe quantum physics with high school equipment
Another thing to do -- when she thinks she gets it, have her try to teach it to someone else. I can't count how often things finally really "clicked" for me when I tried explaining to someone else, especially if that someone else already knew the answer and could let me teach it without interupting, but could confirm that I had it right or wrong.
And, finally, try to find someone to talk it out with. I found one of the best tools for me in both high school and college physics was a dry-erase board. I could stand up, pace, and talk out the problem while working it out on the board where it was easy to correct a mistake and easier to see than on a small sheet of paper. Then, once I worked out the problem, I'd transfer the answer to paper along with a cleaned-up version of the work.
Hope this helps, and sending some PD her way!! I'd offer to help, but it's been several years since my last physics class and I'm not sure how much I remember.

What level of physics is it?