The one nice thing about our engines is that you can remove the sub pan on the oil pan. Try that on a 107 car. You have to pull the engine or drop the sub frame - neither is a lot of fun. :'(
I had the same thing happen to me once and it wasn't pretty. I think some of these pumps went down the line and that step was missed. Earlier pumps don't have this oil pressure relief valve so it's only the late ones. I check that valve to make sure it's tight as a matter of course now.
This wouldn't really be your engine builders fault. Normally, when you get a new part right out of the box it's ready to install, and particularly on something as important as an oil pump. Unless you were familiar with this vague possibility you wouldn't even consider it as a rule.
Your guy really didn't do anything wrong but it would be pretty easy to draw that conclusion. Only a very careful inspection would have revealed why the engine failed although I suppose the loose valve laying in the oil pan might have been a clue.