Can't say I've ever told a patient they couldn't ride. Instead, I usually end up taking too much time with them because we're yacking about our horses!
I recall reading a story about a blind barrel racer. Her parents/friends used to stand at each barrel and bang on it, so she could hear it and know which direction to steer her horse. Now, she wears an earpiece and someone directs her through a microphone. Amazingly brave!
Skyseternalangel: Have you ever tried contact lenses? Horses can't knock those off your face!