She should definitely call her oncologist and let him know what's going on. She could have something brewing, like an infection, and if the pain med has Tylenol, it will bring the fever down. It could also be a side effect of the chemo. From what I remember when I took a friend to chemo, she was instructed to let them know any time she would run a fever. Best of luck.
Pain meds do have fever reducing qualities as well....to check for any underlying infections have her call her doc and get checked out!!
If her pain meds are something like vicodin, norco or percocet, they contain tylenol which will lower fever. Her doctor needs to be notified immediately about the fever.