C.C.
This may seem like it's out of left field, but has she had her thyroid checked? Low thyroid can cause the fluid in the joints to reduce, causing a lot of clicking noises and pain. I went through that myself a few years ago, and my doctors could find nothing structurally wrong. Physical therapy didn't help at all. But my knees (ankles, hips) hurt all the time.
Because I had been experiencing many other issues besides the joint pain, I ended up being diagnosed with low thyroid, and when I began taking medication for that, the joint pain cleared up as well. So, it's something to check. It would be fairly unusual for a young person to have thyroid problems, but it does happen.