Assuming she was honest about what she was offering, where's the problem? She's old enough to make her own decisions. If she wants to have sex with 3 different men per week, or 5 different men per day, or every man whose name begins with 'A', or whatever, why should I care? As long as she tells them that she's offering just casual sex and they consent, she's not doing anything wrong.
Calling her a prostitute is just silly. If that was what she was doing, she'd be charging per customer and the customers would be paying. She's being paid for advertising a business (the website).