Make a partnership/deal/whatever with your client to develop, maintain, and support the software for him, this other person, and anyone else who might be impres

Make a partnership/deal/whatever with your client to develop, maintain, and support the software for him, this other person, and anyone else who might be impressed with it. The software will be better if it works for more people, each of them will pay less, and it could become a minor regular income or a full-on software business for you.