I would just say you can't accept non paying people into the class (that's a policy for adult education classes in my city). But you'll be happy to have her along if she takes the class.
Either she won't want to pay and won't come, or she will pay but it will be clear she isn't being an assistant.
I can see the friends point of. New too. I never do classes because I don't want to go on my own either :P