I don't think women wear belts like we do. I put on my belt in the morning and pull it tight to keep my pants up all day.
Most ladies wear more fitted clothing and a belt is mostly used as a accent. Women's jeans are cut in a way that usually just buttoning them will keep them up. I doubt she will be wrenching it down tight to wear all day.
I'd say talk to her about how she will wear it and what limitations you think the project has and go from there.
On another note my boss taught me that sometimes you have to sell somebody what they want. You can't talk them out of it and if you do, they get embarrassed, leave, and buy what they wanted from somewhere else.