Yeah, dude's dead. Seriously. If I knew my DM was going to make it so that it was "memorable" or "happen the way they intended" then why prepare Disintegrate in the first place? Talk about anti-climactic!
Usually about the only time I fudge as a DM is ifn the party's favor. And that isn't all that often, either. I can easily come up with another BBEG with no-love-lost. But then again, I'm not a member of the DM-vs-players school of thought. So, someone from that school would see this differently. And that's okay.