I think most people understand that there's an implicit agreement by default when you're playing a ranked game with someone online that neither of you is getting outside help during the match and that you're an asshole if you violate that agreement. But I will make the case that even if it's explicitly allowed in the Terms of Service and the victim won't know, you still cannot justify this.
It's not just that you're breaking the matchmaking system in the moment and harming your opponent's enjoyment by giving them a match much harder than they should've had based on their rating. The problem isn't even just that you're adjusting the opponent's rating as if they lost a well-made match, perhaps causing them severe upset by making them think they're worse than they are. The problem is also that you're creating a long-lasting ripple in the game's rating system: since your opponent's rating is lower than their skill now, other people who play with them (or you) will have their rating adjusted inaccurately, resulting in less equal matches for everyone in the commnuity for a long time to come. You're making everyone's experience less enjoyable except your own. It's the definition of ruining other people's fun for your own enjoyment. A Terms of Service that allows the practice doesn't change how human psychology or rating systems work, so it can't make this okay.
For the last fucking time, I know you aren't breaching a contract. But that isn't the whole of moral behavior. A business owner who refuses to hire someone on the basis of gender or race despite ample ability is an asshole, but not breaching any agreements. Just because it's allowed - even by a person that has legitimate authority over the domain - doesn't make it not morally reprehensible.
Smurfing is similar since new accounts usually start at a very low rank, but at least a smurf only harms other people while it's getting up to its proper rating and then after the ripples dissipate everything's right again. Whereas if you get coached regularly (but not all the time) the ripple will never end because you keep producing it.
I don't object in the slightest to "co-op accounts" - separate accounts specifically for a pair of people to play (provided the players create the account when they're new so there isn't a smurf-like rating disparity). These don't mess up the rating system because they're indistinguishable from just one more skilled person playing consistently. They don't give other players poorly made matches and lower their rating inaccurately. Though the real solution if you want to play with advice is to play casual matches, assuming the game has it and the queues aren't dead as a doornail.