I have the exact same issue with my FIL, but he lives with us in our house.
It is a massive issue to me, and I don't think I am particularly fussy about cleanliness in general. I absolutely know that he doesn't was them due to the speed of the visit and the location of his bathroom.
Part of my concern is that I clean his bathroom, and it's grim. He misses the loo quite a bit and doesn't bother to clear up. He also sometimes has blood in his urine (due to a medical condition) and so the urine is quite distinctive. As a result door handles, light switches etc sometime have visible marks - it is truly grim.
He also frequently has urine smelling clothes (I do his washing too), so clearly is oblivious to the hygiene implications.
I have tried everything... sending him back into wash them, disinfecting where he has touched after each use, I have even sat him down and explained how disgusting it is, that it makes me feel sick and that I can't stand to eat anything from the fridge when he has touched it. At one point I even explained that if he didn't improve then I was going to leave his son, and the house would need to sold! Nothing, and I mean nothing, works - he will do it when I a obviously present, but as soon as my back is turned he reverts! Grim