It's just not possible to determine what someone's size is from a height and weight. I've seen people post on here that weigh within 10 lbs of my starting weight, and I wore a size 30 and they wear a size 22/24. Apparently we were built very differently.
If she is sure of her size, I would get what she tells you. If she's unsure, could she send you her measurements? All plus size clothing store websites have a size chart.