You did the right thing. Better than saying OK and then sulking during the "date." If so-and-so can't ditch the posse for even an hour or two, bad sign.
I'm curious if Anon really texted back "take a hike" or something else. Either way, what a ridiculous request from the guy. Talk about socially maladjusted, I think you were spared one incredibly awkward and unsatisfying date night. Lucky!
Maybe he just has really bad communication skills. What he should have said is, "Hey, can I bring my SEXY friend on our kinky date so we can both show you a real classy time."