The cold, hard fact is that the person who played fewer turns wins on a tie.
The fact that your opponent benefited on your turn does not change the fact that he played fewer turns than you.
In general, a player does not benefit during someone else's turn. It can happen, though.
If you played Bishop, and he trashed a Curse from his hand to force a tie, then he'd still win if he had fewer turns.
If you played Saboteur and trashed his Platinum, he could still gain a Province to tie the game and win if he had fewer turns.
If your Swindler replaced his Peddler with a Province, same thing.
Not that you would, but if you pass him a victory card during your Masquerade, same concept.
Also, consider if you play Mountebank and cause his Gardens to go up in value on the last turn.
It's not common, but it can happen.