Fixing the issue is actually pretty simple, I'll help the OP since he clearly needs it.
Only the leaver gets the LP penalty, in addition, he receives the penalty that all 4 other players would also receive. In addition his account now has one strike against it. The remaining players that did not AFK receive a "loss prevented" and are not penalized. If the original offending party were to AFK again, the same thing happens and the account receives a second strike. One more violation and the account receives a third strike and is suspended from ranked play for the rest of the season. If the party AFKs/leaves three games in a subsequent season then their account is permanently suspended from ranked play. No exceptions, no do-overs, no appeals. Three stirkes, you're out.
"But but, life happens", well then make sure it doesn't happen three times in a ranked season.
"But my internet is bad", then don't play with bad internet.
"But my power went out", move to an industrialized nation with reliable electricity.
"But my mom said..." nobody cares, three strikes, you're out.
If you want a competitive game ladder system, there needs to be harsh penalties for infractions, if Riot instituted this idea then Ranked play would clean up in a big hurry and only those that are really passionate about the game mode would remain after awhile. You want to be considered a "sport"? Then do what it takes.
