The only trouble you have is that the wear is in EXACTLY the same place that every single car ever wears, on the outside bolster on the driver seat where people slip into their seat and rub over it. It’s very possible that the wear was caused by someone being heavy-handed when getting in, and even though the mileage is low that could easily be 3500 one mile journeys which equates to a lot more entry/exit moves than 35 one hundred mile journeys.
Personally I’d be trying to be as nice as possible to them to get them to back down, or even just asking for a contribution towards repairs, as I’m not sure there’s a winnable case here if you take it to the end game.