It does seem a little heavy-handed to ban him over what was essentially a blizzard employee screwup. GMs have done similar things many times in the past, sending players vanity items along with their restoration as a sort of goodwill gesture. I assume the GM thought it was a shirt with flavor text, as GMs have proven not to be the brightest crayons in the box on numerous occasions. I would have stripped him and his guild members of any ill-gotten achievements and loot and left it at that, maybe a 3-day suspension for yielding to temptation and doing what pretty much any player would do in the same situation. Like he said, I wouldn't consider it hacking or cheating since all he did was use an item that was created by blizzard and sent to his friend by a blizzard employee.
My 5c (inflation ftw).
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(USER WAS AWESOME FOR THIS POST)AFK: Attack, Fight, Kill! The healer is telling you to go pull mobs.
WTF: Way To Fight! The healer is applauding your tactical genius.
OOM: Out Of Mobs, go pull more.
"Being around you makes me feel nicer [by comparison]."
-Valaheea
"You can't use math to determine (your) spec." - Vomit of Ursin, on rogue class mechanics