How does my sister go about pursuing a case against her back surgeon?

My sister had back surgery two years ago. She had a long recovery, continued to have pain radiating into her back and down legs. Doctor kept saying that was normal. A little over two months ago, she was hospitalized after falling, and was diagnosed with a severe UTI, which had to be treated with IV antibiotics. The bacteria was staph. Infectious Disease department was involved. An xray and a CT scan (apparently standard when someone is brought by ambulance to the ER After a fall) revealed that one of the screws in her surgerical site had moved, and that there may be something going on around the screw. The surgeon (who never came to the hospital to check on her) felt that it was not a problem (very dismissive) and certainly couldn't cause the pains she was having. She was discharged after two weeks to a rehab/nursing facility, where she really struggled to get mobile again. She was there for six weeks. She was discharged and went home mobility challenged.