The best way to reduce the pain of wearing heels in a standing position?

Hi there, I'm trying to help my gf out here. She recently got a new job as a promo girl but she has to wear minimum 3 inch heels and she has to stand up straight most of the time. She's used to wear those kind of heels but not the 'standing in them for hours' part. Especially the of her feet get hurt. What can you do in that situation to get it more bearable?