I've been playing with my friends in low silver and bronze lately ( this is my smurf account), and hot diggity, you can play anything down there and smear if you've got some basic game knowledge. I went 14/2/20+something on Ahri adc. With full attack speed/crit/AD items, not just because I was playing an AP champ in another lane. The other posters here are right; it's all about context. It wasn't punishable for me to select Ahri into that role, the same way it wasn't punishable to pick out Mundo as a support. It's not the best, but it could work. Why not try?
If you feel that he int'd you over or trolled you on purpose by doing things like taking your CS, running it down mid, etc, and generally did not communicate the win conditions of his off meta selection ( which Riot has actually shown to be the punishable offense in the case of OTP Nunu and Singed supports, for example), then those are instances where you can report them. Though it always circles back to the power that you have over the situation before it even happens. If you don't like the match up, you can always dodge.