I was just going over "Knee Stability & Movement Coordination Impairments: Knee Ligament Sprain" in this months JOSPT.
Several references are made to "increased body mass index" as a risk factor- and the fact that it is more difficult to modify than other risk factors.
Numerous studies also support this theory as a risk factor in patellofemoral disorders & ankle sprains.
If your athlete is overweight, address it in your rehab protocol. Personally, I don't tell them they're overweight. I just throw in plenty of spin workouts, slide board work, and sprint workouts in the pool when it's convenient. They'll start looking & feeling better and that will keep them coming back for more.