Did it strip when you were first installing it or after you rode it a few times?
DA lock rings are usually legit, not like those cheap alloy formula lock rings.
How many full revolutions did the lock ring thread on before it made contact with the cog? Sometimes you cant use some cog/lockring combos with some hubs. One time I tried a crappy "godspeed" cog and lock ring combo we had at the shop on my phil hub but I could only get 1 and a half threads out of the lock ring. If I tried to tighten it I would strip it and my hub would be toast.
This isn't usually the case but it can happen.

BTW, I highly doubt that they'll over it under warranty.