Just thinking out loud here... Purely in economic terms, it doesn't matter how far he is from contract end. The fact is, the dollar value of his phone + cash is less than the dollar value of the Thunderbolt. He profits.
If his contract ends, he could re-up with a new phone and sell one of them, profiting even further... or hang onto his upgrade until the Thunderbolt is no longer the latest and greatest. But no matter what, $450 worth of phone + cash is profitable to trade for $500+ worth of phone.