So you'd rather he make another account and have matchmaking queue him as if he's a new player rather than as diamond? I mean, he's going to stomp either way.
The person is the problem, not the game. There's really nothing Riot can do to prevent smurfing or prevent high ranked players from beating up on low ranked players / new players. I guarantee you there are people out there with gold-level skills who purposely throw games so they can stay in Bronze 5 and hard carry whenever they want. These players only have fun when they get 15+ kills and an insurmountable lead. They've never faced adversity in life and need to always win (unless they choose to lose). It explains intentional feeding, etc... The hextech system and champion mastery make it worse frankly (a lot easier to get S ranks against weaker players). These are the same guys that will kill steal every kill in that game (literally just had a guy do that, I only got an S- instead of an S because he as AD top took every blue and stole every kill I set up as an AP mid).
Some people just think the world of themselves. That's basically all there is to it.