1) Was there beer involved in this installation?
2) BE very sure before asking /accusing a friend,especially if they were helping. It's easy to get mouthy/emotional when something like that happens,you don't want to lose a friendship over it unless you are sure..and in which case they weren't your friend anyway.
3) When I work on my engine, NOBODY is allowed in the garage.....I'm serious.... I lock the door. You would be surprised at the silly mistakes that can happen while you are yacking/shooting the bull. Steps get skipped or forgotten etc.
Keep on working bad stuff happens to everyone.