I do a similar thing when a friend asks me a favor that is going to take enough of my time, think the classic "Can you help me build my PC?" or "Something has happened to my computer! Can you fix it?"

If I'm helping then I refuse any kind of money. I don't give guarantees or service like a business does, I don't want money like a business does. I do often ask that they try to learn from the experience, I'll gladly explain everything I'm doing.

The other option is I refuse, for whatever reason that is.

This keeps my choices those of a friend and my responsibilities those of a friend. I like keeping them that way.