I wouldn't be too comfortable with it as I am generally wary about many things. One, because my boyfriend would see my friends naked. Now, if your friends would make the perfect models for him, then I wouldn't want to hinder his work. I would just be concerned about that making for odd situations afterwards. And two, because it would be out of the studio. Now, if I would be there when he were painting them out of the studio, I would be much less wary. (Personally, I need peace of mind.)
Most of all though, you know your boyfriend the best. Make sure to talk to him if you have any concerns. Boys are happy when their girlfriends are to the point.
