I'm one of those pain-in-the-necks who tends to drone on about playing the game the right way, doing unto others blah blah blah. But this guy doesn't sound like he's earned any special consideration, and it sure doesn't sound like he'd hesitate to take advantage of such a situation himself. And, sure, why be a swell guy and let one of the other forces of evil pick him up? I'd exercise the waiver that you've been wisely saving, and sleep well afterwards.
If you ever get a similar email about an accidental Yahoo TRADE acceptance, though...that one I would respond differently to. Yahoo's user interface for Trades sucks; it invites these kind of errors. If you're going to reject a trade, but are being nice enough to include a Comment to your trading partner, it's very easy to hit Accept. Waivers, though...not so easy.