VG,,,, I'd guess he pulled the trap and the foot (and trap) rotted off from zero blood flow. That would also explain the rotting lower leg.
I also know a gubment trapper down in NV who's caught as many if not more Coyotes than your friend. He tells me he NEVER seen a foot chewed. Don't know anyone else that has "actually seen" one either.
I'm not saying it can't happen but I'd lay money that in most cases there are other factors involved like not checking traps and/or non laminated jaws. Not that your friend's a sloppy trapper, but there's lots of em out there.
Again,, anything's possible but I'll remain a sceptic until I see (with my own eyes) a foot laying next to a proper trap that was checked within 72hrs.
One things sure though,,,, if it does actually happen occasionally, it's not near as often as the old joke suggests.
Edit,,,I should add that my friend runs allot more snares than foot holds but I'd bet your friend does too if he's doing ADC work.
Over 1200 Coyotes and not a chewed off foot in the bunch.