What would cause a fever over a week after hip surgery?

So my friend had metal plates put into her upper femur as the result of a hip injury (or something like that). Anyway, she has developed a high fever a week and two days after the date of her surgery. She has been prescribed oxys for the pain, and she was recently instructed to take ibuprofen alongside her pain medication. The fever developed after taking the ibuprofen, and she last took the ibuprofen at 5 am eastern standard time. Would the ibuprofen have caused the onset of the fever, or could there be a more serious underlying condition?