Can you tell us, does he have a nice body? Maybe he is trying to show it off. Some people are just like that, they don't care. If it really bothers you, tell her to meet you somewhere else or at your house. Remember its house, he can do what he pleases, sorry.
I do agree with you though, My daugher and I went to a friends house too and he did not have a shirt on, it really grossed us out!!!!!