Wiring an earth to your car

Due to a lack of time (& effort ) I took my 'new-to-me' car to a very established local towbar fitter to swap the fitted 12N socket to a 13 pin.

How would you feel if instead of using one of the many M6 earth studs dotted about the car he drilled & used a self tapper? I only noticed last night (he did the work yestarday afternoon) & I went back down to 'discuss' it whith him this morning. He was adamant he was correct in doing what he did & would even do the same on a brand new car, he didn't want 'anyone coming back with earth problems - there won't be a rust problem because he used a stainless self tapper', It's a pity my car is mild steel  The gist of what I said was that he was very very wrong & he should've used the factory stud that was a bout 12" away from where was drilled. Needless to say I won't ever be returning ...... and told him why.

So if you want your car drilled, probably invalidating any rust warranty you might have, then use the tow bar 'specialist' that's not far from Burrs Country Park!