I tend to agree. I would only consider partnering up if the partners would need to work at least as hard as me and the concept was already validated and making money already. It really comes down to risk/reward since no matter how much you enjoy the job/project, you still have to fulfill your needs/wants and responsibilities. Obviously there is always risk involved, but you have to weigh up this opportunity in terms of career advancement and your well-being.

He has interest in developing but has no clear plan and as a client he doesn't know all the improvements that need to be made to a application every day in order to make it bigger so he will probably invest just in some small additions to the site like new features and stuff like that

It still really sounds like a bad idea. His intentions are primarily to use the product so at some point it's going to to deviate from what he wants to a more general solution for others.

Maybe he should be an advisor, maybe he should be an angel if he's qualified or have some amount of equity but not an ongoing role etc, or maybe he should just be billed for the work that he wants.

He definitely shouldn't be your boss or anything even approaching equal partner in this venture, and you have to look at the big picture... you could still be doing this crap in 2018.