Cam Sprocket Carnage

Have a friend that has an '02 F150 pickup with a 5.4 and about 130K miles on it. Somehow, one of the cam gears sheared/spun off the cam. With the valve cover removed, you can see it just hanging off the edge of the cam. Is this at all a common issue with mod motors? Any ideas what would cause this to happen? I know very little about mod motors, and he knows NOTHING about them. Just want to do what's right to get this thing fixed properly. Thanks for any help.

We didn't even bother to pull a head, but I'm pretty sure it's a safe bet that some valves crunched. Valvecovers had not been off it up until this point. The motor is going to go to a local machine shop for a rebuild if it's worth saving (none of us have time/mod motor knowledge) so we'll see what they have to say about it once they get it apart. I'll post back with some pics and info once I learn more.