Tyre fitters have Liability Insurance, or at least they should have! Should anything happen to the car, its occupants, or any one else, in an incident that could be directly related to the trye fitment, then the fitter is up the creek with no paddle! good for him, choosing not to fit the tyre
If t all seems a little OTT, then I can say that I know of someone who gave car a bent MOT, and is currently serving a 10 year (iirc) Prison sentence for manslaughter, the defective car was involved in an accident that killed someone, possibly the driver... I can't remember the details as it was some years ago now.
Be careful what you wish for!
Thom