Well, since both the x-ray and the MRI have not shown an deformities, it should be okay to go ahead with the physical therapy. However if it causes moderate-severe discomfort, she may rest it for sometime followed by the physical therapy. You may like to discuss this with her treating orthopedician.
Hope this is helpful.
Your poor daughter. If you feel you need to get a second opinion do so. I had to search over 4 years to find a surgeon who would look past the x-rays and MRI's (all tests including a bone scan were normal) for my ankle. Also see if you can get a bone scan in the past I had a stress fracture that did not show up on a MRI but did show up on a bone scan. Go with your gut. And by the way when I did find a good surgeon and he did surgery my ligaments were very lax from chronic sprains-- even though the MRI said my ligaments looked normal.
Besides a bone scan see if you can get an ultrasound of the elbow-- just make sure it is an orthopedic ultrasound tech who does it because the quality of the ultrasound depends on how well the tech is trained.
See if you can find an orthopedic that specializes in hands because they also treat the elbow. If you can usually your best bet is a doctor from a teaching hospital. Here is a site that should help you find a hand surgeon--
I hope this helps,