Truth Time: If you smurf, you are creating an artificial elo hell
Not a permanent one like some would like to claim, but an elo hell nevertheless.
People of higher skill are not supposed to be a constant force in lower MMRs. This is achieved in many possible ways, like MMR snowballing. (probably the only part of the tier system that is functioning properly) A person of your caliber is not supposed to spend that much time within those tiers.
Likewise, there's also one more parameter. The baddie friend of yours that benefited from this. He is also not supposed to spend time in higher MMRs. And going down is not as well tuned as going up, so he is going to spend some time there, costing games for his teams.
So by proxy of those two factors, your smurfing creates an unnecessary disparity of wins/losses that should have been the opposite outcome. This in turn inflates the time someone takes to reach their true tier, with the exception of those who are already at the top and can keep on doing it over and over again.
You and subsequently your friend losing games outside of his league are turning a 100 games venture into a 200 one for a part of the community.
Just my 2 cents as someone who has also helped friends of mine in the past and decided to stop.
TL;DR: Your smurfing is not harmless.