Well I have short hair now so I don't do anything except put the full face on. I do wear a Buff (
BUFF® - The original multifunctional headwear) underneath though to help keep sweat and gunk out of my helmet. Even when my hair was shoulder length I still didn't put it in a pony tail.
I've seen pictures of women who french braid their hair low. That and maybe wearing the buff as well might make it comfortable. It seems I've seen somewhere some kind of special hair thingies for women motorcyclists.
Maybe this site could help some:
Motorcycle Hair Tips and Tricks for Women Riders
I was also worried about wearing a full face helmet because I hate feeling like my breathing is restricted. But, somehow it ended up not bothering me at all. What helmet did she try? I have a Shoei RF-1000 and it's been great. I would hope she finds some way to make a full face work so that she can keep her face pretty for you!