No need to wait that long, just call them and say you'll fax over the proof. Lawyers don't want to be a party to a lawsuit later over a few thousand dollars if they might clearly lose it.

They will keep some of his money as fees and give some out to clients, so then if they can't get it back from clients, depending on the rules, they might have to come up with it all themselves. No regular attorney wants that, nor do they want the hassle.

Just call them all up and say this money isn't from one of the defendants, it's an accidental transfer from an unrelated third party and they will be getting it back.