Easiest way to find her size would be to get her bust measurment. Take a flexible tape measure (you can find one at a sewing store) and measure all around the fullest part of her chest. If you don't want her to know about it, I guess you could take one of her bras, hook it together, and measure around the fullest (cups) part. This would not be as accurate, but it will give you an idea on size.

Good luck!