This is an interesting assertion from a team-play standpoint.
Like, I get what you're saying, but at the same time, so should everyone else. Which sort of negates the "he's making you look bad" angle, doesn't it? Everyone knows that Pyke is a ks machine, so it shouldn't surprise anyone when that happens. As long as you are contributing to the kills and you win, that should be the overall goal in a team game anyway, right?
Don't get me wrong, I think you're right. Part of the marksmen fantasy is to be that clean-up crew that survives the initial engage and proceeds to clean up on the remaining enemies. Assassins, generally, kind of negate some of that fantasy, and putting one into the support role is sure to affect that aspect of the game. But, again, it's more about the assassin/marksman relationship than it is about Pyke, specifically. Anyone can ks, and people that do more damage are, of course, more likely to do so. Yi and Katarina are just as bad, I imagine, as they are also champs with high damage and resets on kill.
Now, I'd be more concerned about what he's doing to the grading system with regards to kills/assists. If you have a Pyke on your team, it may be much more difficult to secure an S or S+. That's a very real concern, and one that affects the way that players interact with the game in a much more tangible way.