I was told once, waay back in the day when I was cheering in college, that girls are usually more flexible with their right leg (I have found this to be very true) but guys are stronger with their right hand (also true with right handed guys), so girls are taught to stand on their right leg b/c it's easier for them to work on flexibility than it is for guys to stunt left handed. I see complete logic in that. So if I were the forefathers of cheer (herkie? jeff webb?) I think I would have taught my jr. high and high school flyers at cheer camps to stunt on their right leg b/c eventually they'll be college cheerleaders that need to partner stunt with guys.
I find that most group stunts have no problem on either leg and your flyer will adapt quickly in balance, it's the flexibility she will need to work.
Good luck! :)