Personally, I think leaving the mag in was a bad idea. I get your reasons for not being fussed (chamber being empty etc), but if something *had* happened then it would raise questions as to why you had a gun with the mag in, in the SafeZone. It doesn't reflect well on you.
Glad you've learned from that.
Ultimately the fault would lie with the person who pulled the trigger (IMHO).
Chances are that if he didn't check the safety (which is likely, who would pull the trigger knowing it was on Safe?), then he probably didn't think to check the chamber was cleared.
It was an accident waiting to happen, glad no one was hurt!