MumOfSixBirds, I know that this won't be of much help to you, but I lost count of all the friends that have disappeared ever since they found a partner. The ones that have remained, I can count them on my right hand.
I think that just like your long time friend, those people are making a huge mistake. I would never trade my friends in for a partner because, let's just face the reality, most relationships do not last. Sooner or later, I will need my friends just like they need me.
With that said, each and everyone of us has screwed up something, somewhere. While I would let your friend do his own thing, I would not hold it over his head for too long. Sooner or later, he might realize his mistake. Sometimes forgiveness is better than a long term grudge. Sometimes, those who piss us off the most, end up helping us in a dark hour. It can work like that, but we have to keep our door open for it.