Try saying left or right side rather than off side.
You’d have to be stupendously stupid to work in the UK car maintenance trade and not understand both what O/S and N/S mean AND why they are used. I’d imagine that it’s taught on day one.
Port and Starboard would be a suitable replacement.
Spare a thought for dentists..they spend an entire career looking in the mouths of patients and describing the teeth’s location (“upper right 8” etc etc) the wrong way around from where they are stood.
But back to the OP – forget the left/right/offside/nearside shenanigans… they went in for one tyres replacement and two were removed. Regardless of location, that’s one too many.
My aged mother took her polo in for a service at a VW garage in the middle of a highland winter. Advised her the front tyres were worn and should be replaced. She said yes. I saw the car 8 months later for the first time and they’d replaced them with summers. So summers on the front, winters on the back on a front wheel drive car. Lovely. They admitted their mistake and replaced with winters FOC when she queried.