I have to tell that build a pay site is not as easy as staring a video tube on WordPress. You will need to find a payment processor as well as the content management system and they will charge you a monthly fee and do other paperwork but luckily you can also use a platform like ModelCentro which lets you build your own porn pay site easily without doing any code yourself and for free and they take care of the billing, hosting and other stuff and they give you up to 85% of all the revenues you generate.
If I were you, I would also make her do webcam shows and once she gets some following, she can advertise her paysite to her viewers where they can get some content from. It is the way many models make money these days.