Honestly, I think the "fair" thing would be taking whatever Riven would have been rewarded with (any xp and LP) and divided equally between the four players.
I honestly thought my idea was great, but Riot will never implement: If the team lost, the four players get a loss prevented and the Riven losses LP as if they lost five consecutive games. They say it's bad if you have to leave for an emergency, but the way I see it, if it really was an emergency, you'd be lucky if the LP loss was the worst thing that happens to you.