My client owns a dry cleaning business. He serves people who stay at certain hotels in NYC. Some celebrities have been his customers while staying at those hotels. He would pick up and deliver their clothes for dry cleaning, sometimes hand-to-hand. Can he mention that fact and their names on his business website to advertise his services?
He should probably ask the celebrities and their agents, as well as the hotels he serves for permission. It is bad practice to distribute customer information without permission, to say the least. Not doing so and just mentioning the clients without their express permission could put you/the client in a bad position which could damage your/your client's reputation.