Never during a lesson, but twice in my life, both in the last 6 months. In neither did I go completely out, I just couldn't see or hear very well at all, and could barely walk.
The first was for my sports physical, after I'd just gotten three shots. I'm thin, and the doctor said people that are thin can sometimes feel the shot spreading....ugh, I could. Weird. 5 minutes later, while leaving the doctor's office, I started to not be able to see or hear at ALL, I could see black splotches all over, and all the sudden I was really mad for no apparent reason. Swaying....took me to a table in the doctor's office and gave me juice so I would stop my blackout.
Second time I was at a track meet, someone feel into my lane, which threw off my stride, and I fell headfirst over a hurdle. Landed on my shoulder, didn't realize anything was wrong until ten minutes after....ended up being a broken collarbone, snapped clean in half. But I blacked out and had to have my teachers half-carry me to the car! Omg how embarrassing LOL!
Can't help you about what a riding instructor would do, though. I don't take lessons :)