Just let him/her get on with his/her own life. If s/he starts pestering you about it or starts being a bit of a d*ck because you watch it, tell someone.
There's no point forcing it on him/her. I had a couple of friends who were really reluctant at first, I didn't force it on them, mind, but eventually, they got curious. They're not bronies, but they do like the show for what it is.