Montana resident Carol Anne Sowerby said she knew Young from a North Carolina summer camp she attended as a teen. He taught kayaking.
Tearfully, Sowerby said she came to visit the Youngs in the fall of 2006, and she and Jason wound up having sex on the living room couch one night.
She said she was fearful her husband would find out.
"I remember saying we couldn't tell because I didn't want to hurt him or her," said Sowerby, referring to Jason's wife Michelle.
After her return to Montana, Sowerby said she learned of Michelle's death.
Michelle Young was 5-months pregnant with the couple's second child when she was beaten to death in the couple's home on Birchleaf Drive in Raleigh on November 2, 2006. Jason told investigators he was on a business trip in Virginia the night of the murder. Investigators said the killer left the Young's 2-year-old daughter Cassidy with her mother's corpse for hours and her small bloody footprints were tracked throughout the home.
Prosecutors contend the Youngs were having marital and financial problems and Jason wanted out of the marriage. They claim he left the Hampton Inn where he was staying in Hillsville, Virginia and returned to the Triangle to kill Michelle.
The defense says it doesn't deny Jason was a less than model husband, but that doesn't make him a killer.
As her testimony continued Thursday, Sowerby recounted a bizarre incident the day after she and Jason had sex. She said Jason asked to see her wedding ring.
"He swallowed it or pretended to swallow it. I don't know what really happened," she said. "I was told I couldn't get it back until it 'passed.'"
Sowerby said Young returned the ring to her the next morning.
Jason moved to the North Carolina mountains with his daughter after Michelle's death. It would be two years before he was charged with her murder.
Sowerby said he visited her in Montana during that time. She testified she felt sorry for him and he seemed to shut down when the subject of his wife's death came up.
"He didn't really want to talk about it," she said.
Sowerby said she finally broke off all contact with Young at the request of her father who was visited by Wake County detectives. She said she had only recently told her husband about the sexual encounter in 2006.Former fiancé testifies about fight
After Sowerby, Young's former fiancé Genevieve Cargol testified about a physical fight she had with him. Cargol said she and Young were attending a wedding in Texas when he became very intoxicated. She said there had been arguments about Young's drinking in the past, and she brought it up again.
Cargol said Young became very angry and demanded his ring back. Cargol said the ring had always been too small and getting it off wasn't easy.
This made Young angrier, and a physical struggle ensued.
"So hard that I thought that my arms were going to come out of their sockets," said Cargol.
Cargol said Young left her with a cut on her finger and bruises as he took the ring back.
Cargol said she broke off the engagement soon afterwards, but they continued to date for about a year.
Also Thursday, SBI shoe impression specialist Karen Morrow testified about bloody footprints found near Michelle's body.
Defense attorneys say some of the footprints are too small to be those of Jason Young, but Morrow testified some of the prints were larger and made by a Hush Puppy brand shoe.
Morrow also talked about the tire impressions found at the scene - which she said came from a Michelin tire. She could not say if they came from Young's Ford Explorer.
Morrow also talked about a bed sheet found at the scene - which she thought may have had a knife impression. The defense said that wasn't the case.
"It's an impression of some sort and whatever it is, it doesn't match the pocket knife that was in Jason Young's Explorer?" asked a Young attorney.
"No, no sir," Morrow responded.
Young faces life in prison if convicted.