There are some costs that he may or may not have considered. Sails don't last forever, he'll need to pro-rate the price of replacements and think of that as an annual cost. And there's the bottom, he'll probably need to haul and paint at least once a year down there. The haul will cost, as will the paint.
Those kind of things can creep up on you. If he hasn't literally gone over a boat (any boat) and listed every item, along with the annual replacement/maintenance/"oops broke it" costs, etc...He may have missed significant dollars.
If you actually make a spreadsheet on the computer, and list prices starting with the asking price of the boat, what it needs, the cost of that, and the LABOR that it will take...you can get that surprise. Especially if you look at labor as a variable, i.e. what a yard would charge ($150/hour?) what you might do it in, and the time it would take if, like most boat projects, it gets harder as you get into it. Whatever he can get paid for his labor elsewhere--that's a dollar figure to put into the budget.
Personally I don't think "liveaboard" is a way to "live cheap" any more, at least not in the US. It may be a way to make a BOAT AFFORDABLE, but unlike a house, the boat will devalue, so it is just shifting the rent to something that needs lots of maintenance. (But can be real fun on a nice day.)