Hip replacement surgery involves cutting off the hip ball from the end of the femur and machining the femur itself. A hole is bored/reamed into the femur to accept the stem of the prosthesis. I imagine that a femur fracture in the vicinity of the prosthesis could be complicated to repair depending on what sort of fracture it is and how the prosthesis is affected. The good news is that even if extensive work is required, he can probably be semi-mobile with a walker in a day or two. If everything goes well, he might be back on his bike in six to eight weeks. Modern orthopedic surgery is nothing short of a miracle.