There's a reason you can't queue silver with diamond for duo

If that's the difference between your main account and your friend's, then creating a smurf account to specifically to help them climb in ranked is in fact boosting.

While playing duo queue is not boosting in and of itself, specifically creating a smurf account to bypass the matchmaking requirement of similar rank is. You can argue all you want that "that's not the intention," but if it were truly just to play the game with a friend then there's normal mode, which doesn't have a requirement.