I would show him the post and ask him to explain himself.
All we have here is an unsubstantiated allegation; it happens to be an allegation I personally believe, but it's still not reasonably enough to fire someone over. If he was my employee and responded "that's all lies and I wasn't even at the pub" I'd encourage him to sue for libel. If he said "oh shit... yeah, I screwed up", I'd reconsider any important responsibilities I had assigned him (since such behaviour is evidence of not being able to act responsibly) but continue to employ him as long as he could contribute in other ways. If he said "yeah, so what's the big deal?" I'd fire him on the spot and phone the complainant to encourage her to press charges.
But above all I'd wait to hear both sides of the story before passing any judgement.