a size 16 in england is equivalent to a size 12 in the US, and at 5'10" there's a good chance she's at a healthy weight. if you use the hotly debated BMI method, as long as she's under 173 pounds, she could be very healthy. the most important thing, health wise, is that she exercises, eats well, etc. she's not obese by any standard, maybe not even overweight.