At What Point Are Unusual Picks & Trolling Unacceptable?

Recently I have ran into a player who only plays Thresh. He's mostly Thresh Top + Mid every game. Now, I'm not quite against unusual champions playing as laners but.. this person never does well and he's losing a lot more than he is winning (He feeds nearly every game, and he only has a 43% win rate currently). At what point is his "unusual pick" considered a negative influence to his teammates? He's consistently feeding in his games and losing, does this not constitute some form of intent.. seeing how he keeps doing it even though he's seen little to no success?

I did not report this person when I ran into him, because I didn't really care that he was trying it out (I figured maybe he was just good, or having some fun).. but after he was struggling the entire game, I decided to check him out. I then proceeded to see that he does it almost every game and has never really had anything positive come from it.

I'd really love to see some input about this, how should a player handle this situation? Should a person such as this be reported? Is it acceptable? I am truly uncertain on my feelings regarding it and I'd love to understand what other people are thinking.