If they know it works, then they have every right to use it to try to win. I play shit like Vi top, and Jarvan mid. I played Fizz jungle back before it got popular. I've done jungle Urgot in the past. Even AP Maokai Mid has worked well for me. Picking something off-meta is a completely valid strategy to avoid getting counterpicked, I do this often when I'm first or second pick. And I've had plenty of successful games doing all of these!
Off-meta is not automatically trolling just for being off-meta. If it's something the player knows and uses effectively, then there's nothing wrong with it.
And having the occasional bad game is the same whether you're playing meta or off-meta. It's only your perception of the player that changes, because they did something off-meta that you consider to be "wrong" and "stupid" already, so you plug it into your confirmation bias that off-meta is equivalent to trolling and feeding.
That's not their fault, and you are responsible for harassing a teammate for their mistakes regardless of whether those mistakes were 'doing something off-meta' or 'overextending,' or 'ganking at a bad time' or 'missing CS,' or 'stealing a kill' ... etc.
It's not your place to tell someone else what to play. Doing so politely is fine, but throwing a fit or asking to report them when they don't listen to you is absolutely being a dick.
It's possible to be a dick towards a person being a dick. Gosh it's funny how that works. Just because you think someone else is being a dick does not give you the right to act like an asshole towards them.