You'd have to be standing till the end, i.e. wait for the clock to run down and see the "winner" indication.
If so. drops out and you turn away, there is no winner (credits to the company), and if he eventually comes back while the clock is still running, he will be announced as the winner - look on his profile!
But, AFAIK, the match will be counted as "not completed" for both in the relevant stats.