Replacing a 2 bladed fixed with a 3 bladed means either pitch or length needs to go down or he will overload his engine and not get reasonable performance. He could modify the pitch and help that, but he'd had to be careful and know what he's doing. The luddite reference means that's not likely, so -
He needs to consult with a sales representative of any one of the companies offering propellers.
To answer you question - many Hunters have been retrofitted - search the forum at
Hunter Owners.com for owners sailing production sailboats
Chuck - former hunter owner.