5100 snap ring failure

Hmm. Yeah that's a tough one. I'm inclined to say this is on the shop, not Bilstein, since they assembled the shock. They should check the ring. But there is always a risk to bringing your own parts in because the shop doesn't know the history on the part to guarantee it, and it creates a grey area like this when something happens.

I would politely go back to the shop again and see if they will own up to it, or at least go halves on the repair/ replace costs.