K.M.
I was smaller than that in middle school and got my period in 8th grade (I'm only 5'2" and 100lbs now as a grown mother of 3). If she's healthy and just a petite person, she'll be fine. I think it's more for the girls with eating disorders (I had a friend who was anorexic and didn't have her period for years). They also say skinny people have a hard time getting pregnant. I got pregnant with all 3 kiddos on the first try. Again, if she's healthy and just small, she should be fine.