I dunno... I've decided never to send emails in reply to something hurtful until 48 hours afterwards. I've seen emails go wrong far too many times, and I read a study that 80% of emails are misinterpreted. I think I have to agree that thinking carefully first and writing slowly is a good idea.
To the original post though, I'm sorry your old friend disappointed you, I know how much it hurts when someone turns out to not value your friendship as much as you value theirs.