One of his transactions was on the 0.8 chain (during its short life, and the other was on the 0.7 chain. So it's better to think of it as a transaction being in

One of his transactions was on the 0.8 chain (during its short life, and the other was on the 0.7 chain. So it's better to think of it as a transaction being invalidated rather than double-spent, because in the end there were no outputs that didn't come from an input. This is using semantics to try to minimize the severity of what happened. For a time, the world thought there were more bitcoins in circulation than there actually were. Someone thought they were holding BTC which turned out not to exist, since after the chains were merged, those extra BTC just ceased to exist. That isn't a big deal if both sets of BTC were sitting in a holding account somewhere. On the other hand, it IS a big deal if (as is the case in most financial transactions), the parties exchanged another item of value (say, $1000 cash) in return for receiving each BTC transaction. One of those parties now has -$1000 and zero BTC to show for it.