See the help topic "Incorrect Stats in Live Scoring". Final statistics are not official until all stat corrections have been applied, usually a day or so after the last game in the scoring period. If it was my league, the stats speak for themselves, so whatever the correct final stats are, that's what they are. In my league Team B is the victor in the above scenario, especially since Yahoo! rules explicitly say they can apply stat corrections after the fact. As Yogi said many years ago, "It ain't over 'til it's over".
I think your friend got screwed but White Mountain is right in what he says. To be honest my opinion is that the bet should be called off. Both side have reason to believe they won and friendships have been lost for less.