http://www.love2pedal.com/ carries a lot of sizes for the plus size crowd. Perhaps she can try some shorts from there. Stores don't generally carry sizes beyond XL for women.
Some brands and models (don't know if on that site) have ones that dont have an elastic band per se, but just have the front built so that it secures things without having a pinching band.. Bibs are an obvious solution, but I've found the selection very limited for women, and the issue with using the facilities would be one I'd not want a new cyclist to have to deal with if they are already unsure about lycra clothing.