Just remember, if the seat belt is on, the roll bar should be up. If the roll bar is down, you shouldnt wear a seatbelt. In case of a roll over, you want the roll bar to keep the mower off of you if you are strapped in. If the roll bar is down, you would rather be able to jump clear before the mower rolls on top of you.I ask her politely to make sure she wears it. It's a habit from my work requirements when on equipment.
Same rule applies for tractors.