Persistent fever with urinary tract infection needs to be investigated. The first thing is to make sure the E. coli is sensitive to the antibiotics being received. The next thing to do would be to make sure there is no involvement of the upper urinary tract by stones, blockages or kidney infection. Sometimes, imaging studies like an ultrasound or CT may be needed as well as blood tests to see if the body is mounting a response to the infection. Keep her well hydrated and have her seen by her physician and also discuss these suggestions with your doctor.