Lawsuit would be for modifying their code without permission.
The catch though is that the file nVidia replaces isnt made my FM anyways so its really a moot point there. They could make the case that the DLL is acting like the "MMO Glider Bots" that Blizzard dislikes and modifying the output of the program.. which is partly true, but really if FM wanted to take legal action they'd have done it a while ago.

