First I would try to talk her out of her plan. When you can't do that I would tactfully explain that you will do your very best but your are very concerned that your best won't be good enough. After she assures you that it will be suggest she give you some sort of theme. Songs with "Summer" in the title, songs with "Star" in the title, songs with "Dream" in the title, etc. At least that will give you some freedom to find songs that will sound good.