Don't get me wrong guys, i'm not defending the shop in any way whatsoever. I think what they did is compete stupidity, and yes they should have talked to the owner before hacking everything apart.
But what I'm saying, is that in the eyes of the law, the owner may have no case. This is because the act the owner wanted the mech. to perfrom is illegal. You can't enter into a legally binding contract, when the terms of that contract are illegal. So, the court may find that the mechanic could reasonably assume that the customer wanted to hack it all up to high heaven so that he can get the X in there and still have cats ( which would be the only "legal" way of "installing" the X). Also, compare it to what nickthegenius said about the guy trying to sue hydroponics retailer for wrecking his weed stash.
Again, I'm not saying the mechanic was right, he wasn't, he should have talked to the owner first. Im just saying the owner might not win a court battle because of all the legalities that have to be considered. You have to remember, the law is not common sense, it is a very tricky subject, and courts often make rulings that would defy common sense because of some legality or technicality.