This is absent from the rules for a reason. The guidelines for private messages are found here:
Rules for Fair Use under rule 7.
If the accuastion remains private, then there's no problem. However, it becomes a problem if
Rules of Fair Play are broken - see Rule 17. If the player making the accusation threatens to report you for the purposes of blackmail.
What it boils down to is that, in private messages, the accusation is not being used to affect the game. If it is made public, without being reported, then that's an issue. Of course, I'm not a mod, so I couldn't say if it has been reported or not... and I probably wouldn't, anyway, while it was being investigated. What you receive in a private message it pretty much left for you to deal with, unless it's something extreme.
It might have been better to raise this privately with the Mods group via PM. There's a Cheater-hunters group and a SITE HELP! group you could have raised it with.