John "Herb" Friedlund was convicted of theft, and the 79-year-old faces up to 10 years in prison when he is sentenced.
The Spokesman-Review reports Friedlund also faces federal charges of money laundering and failing to file income taxes on money taken from the retirement account of Frances Swan.
A deputy found Swan living in squalid conditions and asking for food last spring when he was investigating starving horses on the property where Friedlund also lived. Swan now lives in a nursing home.
Court records say Friedlund spent Swan's money on tractors, snowmobiles, more than 300 guns and young men he met on a website for "gay sugar daddies."