Knee Problem need advice

[JarHeaD316]

Hey my friend recently just told me he has a bump on his knee and he went to health services on our campus and they said that it could be fluid build up or cartilage damage. He smokes regularly, doesnt exact follow a healthy diet and is very sedentary (sp). I dont think hes broke a sweat in over a year. Im trying to get him to change his life style but he just doesnt budge, Im thinkin this injury may be for a greater cause.

my Question is however, what caused this injury if he doesnt do any physical activity (which may actually be harmfull as well)?