Most hamstring avulsion injuries are related to skiing -- water skiing, cross-country skiing, or downhill skiing. Falling or slipping with the hip flexed forward and the knee straight is just too much stretch on the hamstring muscle. Violent overstretching of this type pulls the tendon right off the sit-bone on the pelvis.
Surgery is usually recommended. The sooner the better for the best results. The patient comes out of surgery with an elastic bandage to support and protect the surgical site. Some surgeons use a leg immobilizer; others don't.
The patient uses crutches at first with no weight-bearing or just light (toe) touch to the ground for balance. Sitting is not allowed for the first two weeks. Stretching the hamstring is not allowed for at least four weeks (sometimes longer). Full weight-bearing is allowed gradually.
If there's no infection or problems with wound healing, pool therapy can be started four weeks postop. A physical therapist will work with your grandfather. You will probably be able to observe and even help with the program.
Range of motion exercises begin four to six weeks after the operation. A strengthening program doesn't begin until several months after the procedure. The surgeon and therapist will advance the program appropriately.