It depends, what is her bmi... body mass index? You can calculate it here:
http://www.nhlbisupport.com/bmi/
Height and weight variables are different depending on body shape and such, since we are all built differently. Is she pudgy, have a double chin, baby fat on her cheeks, does she have belly fat that hangs over her pants? Are these things she's concerned about? If so, then yes, she is probably over weight, and that still depends since a lot of kids with baby fat tend to thin out as they get taller. If not, then she sounds good. Is her friend very petite and thin? Perhaps she is comparing herself to someone who is more petite than average and doesn't understand that. Was it her best friend that called her overweight and that is how all of this started?
I would certainly talk to your daughter about having a positive self body image and keeping it healthy. That means not being rail thin, or overweight, or having perfect proportions, but eating healthy, getting good sleep and staying active.... and help her see that other people saying she's something, doesn't mean that they are right.
This article discusses what a healthy weight range is, and how it changes during this time in kids lives:
http://kidshealth.org/teen/food_fitness/dieting/weight_he...
According to this chart, 52" and 79 lbs is the average height and weight for an 11 year old girl, and 47 inches and 63 lbs for a 9 year old girl. So, it looks like your daughter is right on target and the other little girl is on the small side.
http://www.disabled-world.com/artman/publish/height-weigh...