I don't think you should have. For one, maybe the player had discussed it with their team first and they were fine with it (okay, doubtful, but you don't know that). Just because someone played a champ out of their main role doesn't necessarily mean they're trolling, only if they make the experience unpleasant for the rest of the team. Maybe that player was just having fun (although they shouldn't have done it in ranked), or maybe, as Dynikus said, they had seen a pro player do it and wanted to try it themselves. Ideally you should do that sort of thing in bots or normals, but not everyone is going to.
My belief is don't report someone on the enemy team unless you know entirely what's going on.
On another note, I tried ADC Soraka once in bots. I would never, ever do it in pvp, but it was good fun.