There are some brands that are now more forgiving in the stretchy fabric and still look professional. I found some pants that are stretchy material, but still look like business pants and goes great with a blazer. The key to a blazer (if her arms/shoulders back are bigger) is if that fits. Some make them with stretchy material in the underarm area so that is more giving.
I'll look to see what brand pants they are....can't remember off hand.