Before she actually does this, she goes to a major on-line company like Travelocity and sees what price they can give her on a car. Once she knows what the low price is she goes to Priceline and makes an offer even lower by several dollars per day. She usually finds a mid-size for about 14 dollars per day. Her tip is to bid low, as you can always increase your offer. It just seems to take a few hours sometimes.

I hope this helps.
References :
http://www.travelocity.com
http://www.priceline.com