I haven't heard anyone mention the costs of moving. Unless he lives in a transient hotel with just the clothes on his back, there are costs involved beyond just a Greyhound ticket.
Does he own a home in NY? It's not enough to just rent a furnished apartment in Florida and yell at the top of your lungs "I do declare myself to be a Florida resident." You have to abandon your NY residency. Even if Florida considers him a resident, NY can also consider him a resident at the same time.
To make a convincing case that he had abandoned his NY residency, he will probably need to sell his house there. Which means real estate commissions and other fees. It could possibly mean tax on any profit from the sale (if it exceeds the exclusion amount). Even if he decides to take a risk and not sell, there is the expense of rent for a year in Florida. Or if he buys a place to live in Florida, there are expenses, too.
And there are the expenses of moving back. He might have to sell a place in Florida and buy a place in NY. Real estate prices may have changed. He may make a profit, he may take a loss. In the worst case, prices in FL may have dropped and prices in NY may have increased.
I notice that the person who suggested that there is a federal deduction for state taxes got red, but this is also a consideration. Any lost federal deduction will decrease the potential profit to be made by moving. (The deduction decreases the out of pocket cost of paying tax to NY.) Of course, we can't tell what the actual benefit of a deduction would be because AMT will probably come into play here.
And how certain is this profit? If he waits to sell until after he moves, could the value of the investment decline by an amount exceeding the NY tax?
You say he is self-employed. Will his clients follow him to Florida? What will the costs of re-establishing his business in FL be? I don't know the nature of his business, but if he continues working in NY while nominally living in FL, he'll have a harder time proving he abandoned his residency there.
Yes, it may be profitable. It's hard for me to imagine myself moving back and forth for less than the cost of the taxes he will save, but I'm not him. But I've seen no mention of any other costs involved in this scheme than just the tax savings.