Well, this one will have to fall under the "well duh" catagory.
Long story shortened: my friend has never had a mower with his tractor although he does have front loader and backhoe attachments. As a result, he has never had a need for the PTO. Apparently, the PTO lever never got exercised to the point where it would spring back to the OFF position. The operator, or a tree limb, moved the lever off of the start safety switch. I could have fixed this one over the phone.