Given that the doctor says it's 'mental' has he suggested anything that would help her? Is he suggesting psychotherapy? How about alternative therapies like massage, meditaiton, etc. that have proven helpful with pain. Many hospitals have what's called mindfulness based stress reduction classes that can help people for whom traditional therapies have not been successful.
Physicians get frustrated when they can't help but too often they go right to the 'mental card' when they've either run out of options or have stopped looking for them.
Is this her primary physician or is she seeing a pain specialist? If she's not seeing a pain specialist - someone who's knowledgeable in the many different ways to treat pain - she should probably try to find one. Living in pain is not an acceptable option.
Good luck!