How else is matchmaking supposed to handle it when people of disparate skill team up? Should they match on your mmr just because you're party leader? So your buddy can have a good old fashioned pub stomp? For as many matches in a row as you choose to stay online with him?
The long and short is that games that encourage friends to play together always create this situation, and it's always been a no-win scenario. Putting together teams with players who are not very close together on the skill spectrum is a guarantee that somebody somewhere is going to get screwed. It's gonna be you or a bunch of strangers who did NOT sign up to get destroyed when the matchmaker let a fox into the hen house. You, on the other hand, consciously chose to party up with this guy and that means you chose to play his lobbies, and it means you can choose to leave them at any time.