Add up all the LP gained and Loss for both teams and that person who AFK'd takes that lump sum as a loss to their LP.
There are a ton of problems with this idea, the first and foremost being one that GatekeeperTDS has already highlighted; any Ranked system which fudges LP gain and loss for any reason other than losing or winning the match will wind up fundamentally broken.
Another issue, of course, is that your idea is outright overkill. You're not just suggesting the AFK takes the LP loss of their team; you're suggesting that the AFK take the total potential LP loss from both teams. And quite frankly, that is absurd. How would you like to lose six times as much LP for simply losing a game, solely because the game arbitrarily adds the potential LP loss from the winning team to your own LP loss?
I'm doubtful you'd want that to happen, so why try to tack it onto your proposed punishment?
And, on top of that, though this goes hand-in-hand with the first point, any system which mitigates/removes LP loss will inevitably encourage players to game the system to maintain their rank. And that means people queuing with people who wouldn't care for the LP loss to take the fall for them, or trying to bully someone to the point of leaving the game so that they don't suffer the LP loss for a match they couldn't win.
And that is not the kind of behavior anyone should want to encourage.
...and people just rage quit forcing me to lose LP should not be acceptable.
It isn't acceptable, but League of Legends is a zero-sum game; one team has to lose, and one team has to win. And consequently, you're going to have to deal with losses and situations that are beyond your control. It's inevitable that you're going to lose matches - and LP - so you have to be able to soldier on from those losses and keep climbing.
