Personally, I'd have let him play when the singer first asked. You'd have been right there with him at that stage. What damage could he possibly have done with you standing a foot away? Providing he played and didn't abuse.....you might have had some fun with him. But that's just me.
That said, as for playing after you'd already knocked it on the head? Definitely not cool.
What's the best cape for running away from a gig?