Usually the answer to this one it tough luck, he should have been more careful. I find that in this case the other owner is trying to make an excuse for not thinking a trade out and accepting it on "accident."
However, since you mentioned it was among friends, it may be a different case. I am guessing money is not involved, so it is not as big of a deal. If your other friends done mind, I suppose it cant hurt to reverse the trade. BUT, i dont know if that is possible in Yahoo leagues, and I dont know how you would go about that process. I think the commish would have to have the ability to edit both teams, which is an option to select before the season (I believe). I am not too familiar with Yahoo leagues because I am not very active in them, but something might be able to be worked out ONLY if everyone is ok with it.