Unfortunately a hooker's idea of personal hygiene is probably not what one would wish. I doubt if this is an STD as such, but he could have caught a more common-or-garden infection from her thus getting diarrhoea. I can only hope he learns his lesson, and waits for a real girlfriend. If the diarrhoea goes on much longer, he could take himself to a STD clinic for an anonymous check-up just to be sure.
