Although the exact cause is not certain, it actually is felt to be more of a problem where the knee cap tendon (the end of the muscle) attaches to the leg bone. Remember that children's muscle and bone (skeletal) system is at risk for irritation and inflammation when kids are growing taller the fastest (peak height velocity). Studies show this may be due to biochemical changes, which happen most often during puberty (in boys between the ages of 10 and 15 years and in girls between the ages of 8 and 13 years).
The symptoms of Osgood-Schlatter Disease include pain over the bump just below your knee cap, usually worsened by exercise that involves repeated bending and straightening of the knee and by the direct force on that area. Treatment consists of limiting activities, putting ice on the area, and using something for the pain. If the knee pain occurs during running and jumping, cutting back on the intensity of training during this period of rapid growth should help in relieving the acute symptoms.
With contact sports like football or soccer, the use of a kneepad to protect the tender bump can also be helpful. More aggressive treatment like surgery is rarely needed. The disease is self-limiting and ends when the bone in that area stops growing. Have your friend speak with his/her doctor to make sure this is the right diagnosis and how best to treat it.