While 11 is on the younger side, it's not super young. We have seen onset of menses in girls as young as 8, occasionally, but I think the standard definition of "normal" is 10-16.
One thing for her to be aware of: there is evidence to suggest that girls who start prior to age 12 may have a higher risk of breast cancer in the future. This is something she may want to discuss with her doctor when she gets older. (It seems to be the case that the longer your body is exposed to estrogen, the greater your risk of breast cancer. We know that early menarche (onset of menses), late menopause, having fewer pregnancies, and not breastfeeding can increase the risk of breast cancer.)