Sometimes when cats "tinkle" outside of their litter box, they might be doing so because another cat is intimidating them or bothering them while they are trying to use the box.

When a friend's cat spent a week at my apartment recently, my cat Leon was unhappy to share his litter box (although my friend's cat is his sister, RC, and they were lived together for most of their 5 1/2 years). Anyway, RC took to tinkling in my houseplants or on the floor . . . and no wonder, because whenever she was in the litterbox, Leon would be outside the box, sticking his head in and bothering her.

So maybe your friend could get this cat a box of his (?) own, or watch to see if he is being intimidated by other cats while he's using the box?

Hope this helps --