This is how LP should be balanced with AFKs.
If someone goes AFK on a team and they lose, the AFK should lose the full amount of LP, the winners should only gain 1/2 LP and the losers should lose 1/2 LP as well. The reason for this is because, even though it's a 4v5, it's STILL winnable and a team winning against a 4v5 does not deserve to gain as much IP as well.
Now if the team with the AFK wins, then they should receive 1.5x LP because that team WORKED their ass off for it, and they deserve it dammit. The AFK on that winning team should will not gain any LP. However, if this happens within a certain timeframe, he will lose the base amount of LP(without the 1.5x multiplier) that team gained,. It's a win-win for everyone.
If this AFK, rage quits multiple times within a certain timeframe, he will lose 2x the amount of LP if his team lost. I understand that some people have connection issues or have a life. If you know that you have shitty internet or that you have a job that requires you to be on call, it's simple, don't play ranked. Do not ruin your team's chance of winning just because you wanted to see if you can win with 900 ping and disconnecting all the time.