A firm run by Ken Fisher, the billionaire author of best-selling “How To Smell a Rat: The Five Signs of Financial Fraud,” is being accused of facilitating a fraud by employing an executive who fleeced a client out of $713,000.

Long Island businessman Tony Lin says he invested on the advice of a Fisher exec, Mark Swanson, who, court papers claim, kept the loot to fund his gambling addiction.

In a suit in State Supreme Court, Lin accuses Fisher Investments of “[permitting] Swanson to perpetrate unlawful acts from [their] New York offices . . . particularly shocking in light of the published writings of Ken Fisher.”

Fisher is a finance whiz and Forbes columnist who’s written eight books, including “How To Smell a Rat,” which Publishers Weekly said, “helps [readers] invest safely and identify financial scams big and small.”

Lin alleges that Swanson, a former VP at Fisher’s Private Client Group, solicited him to invest in the “Stellar Fund.” Lin was later told Swanson had been terminated. Then, “Swanson admitted . . . [his] entire investment . . . was gone and that the Stellar Fund was a fraudulent entity created . . . in order to fuel Swanson’s long-term gambling addiction,” the suit says. It blasts Fisher Investments’ “willful blindness” and “unlawful and negligent conduct.”

Lin wants $2.1 million in damages. He also has a federal suit pending against Swanson. His lawyer, Mark W. Smith, said, “Perhaps Ken Fisher’s ‘How To Smell a Rat’ should be required reading at Fisher Investments.”

Fisher rep Bill McBride said, “This suit is a pack of knowing lies. Fisher Investments uncovered Mr. Swanson’s fraud — which had nothing to do with Fisher Investments — and immediately sacked him, notified law enforcement and informed Mr. Lin . . . Mr. Swanson and Mr. Lin had an existing relationship long before Mr. Lin hired Fisher Investments or Mr. Swanson was employed here. This case has zero legal basis and is the result of a failed attempt by Mr. Lin’s lawyers to squeeze Fisher for money.”