My mother has this, thanks to a difficult delivery when she was born. She can't raise her right arm above the shoulder, and I believe that her right arm may be *just* a smidge shorter than the left. It's not impacted her daily life (however, she doesn't ride) other than having to ask me to get things off of high shelves. Of course, she's 5'4" and I'm 5'10"...
I do know that she recently worked with a physical therapist for something else, and they took the time to work on her right shoulder as well - she managed to increase her strength and range of motion in that arm. Hope this helps a bit...
Is the entire right side of her body affected, or "just" the right arm?
My client has slight deficits on the left side affecting the arm and leg, sometimes with pretty severe tightening of the neck & shoulder muscles on that side. She often cannot feel what her left side is doing, or where her arm or leg is positioned. That's probably the worst part. Switching to a Western saddle from a dressage saddle has helped her a lot.
I have told her I have able-bodied students with worse asymmetries than she exhibits.
Growing up, my Mom often asked me to reach things on shelves. No Erbs Palsey, but she was a petite 5'2" and I'm 5'4" with gorilla arms!
My daughter has Erb's Palsey. It affects her left arm. She gets a lot of tightness with it and if she doesn't keep up with her stretching it can become quite contracted. If she stretches regularly, you really can't tell which arm is afflicted. She is supposed to use it more than her right arm to keep the muscles strong. I do find that riding is one activity she can do that does not make her feel self conscious about it. I make her do stretching exercises when she is in the saddle, that way it seems like more fun.