If a battle is interrupted because of a real-life crisis, there is no way of automating the task of "guessing who would have won". So automatic destruction of the leaver's ship is inappropriate. It would be fairer to call it a draw, with neither ship being destroyed.
But instead FDev have provided a means of quitting which hands a significant advantage to the remaining player - 15 seconds of unopposed action. Seems more than fair to me.
And if he fails to destroy the ship in that time... so what? Why should it matter? If the other guy quits, you've WON. And if you were trying to stop him from doing something specific, like delivering a cargo or attacking another ship... then he won't be doing that anyhow, so well done! (OK, a bit annoying if you're a pirate, but you could pick a passing NPC instead).
If the other kid wants to take his ball and go home, then so what? it's only a game .