Ina Thea Kenoyer, who was arrested on 30 October, has been charged with killing Edward Riley Jr in Minot, North Dakota.
The New York Post, citing police records, said Mr Riley became ill when he met his lawyer on 3 September to collect an inheritance of around $30m (£25m).
Kenoyer waited until the following day before calling emergency services and when paramedics arrived they found the 51-year-old unresponsive.
Mr Riley was eventually taken to a Bismarck hospital where he died on 5 September.
Post-mortem results determined that the cause of death was poisoning.
“This case was extremely complex,” Investigations Commander Captain Dale Plessas said.
“Thank you to everyone who provided us with information that helped our investigators piece this together.”
Kenoyer is charged with Class AA Felony Murder and is being held at the Ward County Jail.
The investigation is ongoing
