I think it's pretty simple, you do exactly what you've done up until the point.
If you have no rules setup to address stat corrections then I don't see how you can institute it because you have a whiner.
I would suggest you simply say "we don't know that this hasn't happened in the past and someone lost (or gained) points they shouldn't have. It would not be fair to the entire league to institute a new policy mid-season. I feel your pain (give Clinton thumbs up and bite lip). That is a definite thing that we need to address in the off-season
MFL has a separate page that shows all stat corrections week by week and asks if you want to implement them as commissioner. Here's a link (hope it works):http://football7.myfantasyleague.com/2011/options?L=30663&O=80&W=1
If any of these weren't made effective in your league then you have a precedent. You also have the issue of 'which site is the official stat recorder'. If you're using something other than the Yahoo site you're using you are asking for trouble IMO as it will have to be done by hand.
Good luck and please update how it turns out!