Follow-up: The mechanic should have a brochure or boilerplate guide for basic auto care chores like measuring your tire/tyre wear. This sets up the power move of “we give this to ALL our customers”, thereby avoiding any call-outs and giving GP’s ex-girlfriend the cover she needed. Instead the mechanic was probably trying to be helpful, but wound up ruining GP’s day in the process by being unintentionally insensitive.
Be a bro: don’t set other people’s partners off.
I think there was little chance it was actually about not knowing how to check tyre wear.
Also, every tyre place I’ve ever been to has had how-to check tyre wear printed in huge posters on every surface available, it makes it easier for them to sell more tyres. This is on top of it being a requirement to get a driving license in my country.