If the nbr does not have anyone coming over to his home as in clients then I doubt your city will do anything about it no matter what it looks like to anyone else , SO heres the deal does your city have a ordinance that spells out the number of visitors that come to a home for a business reason as a trigger point for city when a city can declare a home to have so called business traffic. EG years ago I met this really cool lady who lived in Edina Mn , and ran a weaving business custom making wicker type furn & etc , she had told me as long as she had no more than 2 visitors a day related to the business she didnt need a variance and was not in violation of city ords. SO talk to your city but know this, if your nbr has any hint ever that you may be conducting business at home you too will be watched.