Most schools won't let a "free lancer" just walk in and use their planes. They will say it's for insurance reasons. You might be able to talk them into "hiring" you if you bring your own student, but they would only pay you the going rate the other CFI's get.

There are a few clubs/schools that are set up well for free-lancers. They will put you on the insurance and allow you to be an independent contractor. But, it's pretty rare to find a place like that.

You'll just have to look into how your local school is set up. Perhaps you or your friend could find a private owner that would rent you his plane for a while. The insurance will go up quite a bit but it might be doable.