Jen,

I understand that you don't want to upset this guy but having groomed his cat, I would ring amd ask your local rscpa to look into the cat situation, I'm sure you can report a worrying situation without revealing your name.

If it turns out the cat was a rescue cat no damage done but if the owner can not cope (a more likely scenerio) then they will rehome the cat to a better home.

If I had to take such a badly matted cat to the vets, I would be very sure to explain why the cat was in such a state !!!