Need to convert men's pant sizes to a woman's?

I want to buy a pair of men's insulated snowboard pants for a gift for a woman, but I don't know how to figure the sizes. The woman wears a size 14/16, but has a thick waist for her size and height. She has a 33-inch waist and 42 hips. I don't understand the men's sizes. Some brands only have sizes like "X, or XL", or sizes for men's waists, like 36, 38, etc. What size do I get for her?