I did multiple outcalls to shared households, usually finding out about the sharing part, just as I arrived. While I didn't really mind, and to me it felt safer when I could hear a grouop of loud Italian girls begind the wall, I think some people might not like the idea.
At the end of the day, you never know who is in the house anyway, he might not even mention and the flatmate is sitting quietly in his bedroom. At least he told you about the guy.