Part of me thinks it's their fault for putting that part on there, but the other part of me knows that not every single person working in every single shop knows everything about compatability between different hubs and lockrings. Especially if you were bringing in parts for them to assemble for you, they probably would have assumed that you knew that the parts would work.

How did it strip? It sounds like the problem didn't happen until after you left the shop; which means when they put it on it should have gone on fine, right?