It's a simple reality: if you are collecting money by doing something, from a tax point of view it is almost certainly a business; even if you casually think of it as not-a-business.

But this person, if they put the brakes on the conjecture and so on, and instead go talk to an accountant, can probably get it all taken care. It will take work; it will probably be necessary to refile the last N years of taxes with the appropriate form on which to list both the business revenues, and business expenses (of which it sounds like there were plenty). He may even get some money back.

Of course, doing that work is much less fun than venting.