If your're going to fight it, get another mechanic to inspect the engine and put in writing what he thinks caused the problem. See if he'll agree and write that "running rich" for 1000km won't cause this kind of problem. And then think how you're going to prove it was only 100km. Race car teams wouldn't purposely run their turbos rich if it was going to cause the rings to break down in a short period of time, or would they?