CBS News has learned new details in the mysterious case of missing Colorado mother Kelsey Berreth, who disappeared more than two months ago. Her fiancé, Patrick Frazee, is charged with murder.

Krystal Lee, an Idaho nurse, pleaded guilty Friday to tampering with evidence in the case. A close friend and confidante of Lee says her friend admitted to helping Frazee try to hide Berreth's death.

"I will just tell you Krystal was very, very scared," said Michelle Stein. "Krystal's a very level-headed, kind, fun-loving, happy-go-lucky person. She's a tough cowgirl. But she was absolutely scared and extremely upset."

Krystal Lee has known Frazee for 15 years, and the two previously dated. But Stein says it was only recently that Lee discovered Frazee had been engaged to another woman (Berreth), and that the pair had a child together.

When asked by CBS News correspondent Nikki Battiste if she thought Frazee is a murderer, Stein replied, "Yes. I do. I think he's a very dangerous person."

"Has Krystal ever said that she's been fearful of him at any point?" asked Battiste.

"Yes."

"What did she say?"

"She did tell me that she has never seen that kind of evil in anybody," said Stein.

Berreth, a 29-year-old pilot, vanished on Thanksgiving Day in Colorado, and was last seen in a surveillance video shopping with the one-year-old daughter she and Frazee shared. Investigators say three days later, Berreth's cell phone pinged nearly 600 miles away in Gooding, Idaho, about an hour from Lee's home. Two text messages were sent: one to Berreth's employer, saying she would be away for a week, and another to Frazee.

On Friday, in a court plea, Lee admitted she dumped Berreth's cell phone just two days after Berreth's death. "I had no right or authority to move the victim's cell phone," she said.

Lee also claimed she knew Frazee had already killed Berreth.

Stein believes Lee has "very, very good reasoning for whatever it is she may or may not have done."

"What was the reasoning?" asked Battiste.

"I can't really comment completely, but fear, and legitimate fear."

"That if she didn't comply with Patrick, he might kill her?"

"Yes, that's what I believe," Stein replied.

She says Lee and Frazee, a blacksmith, met after high school at a ranch and bonded over horses. She would not comment on whether the two had a sexual relationship.

Frazee is currently in jail without bail. Authorities believe he murdered Berreth at her home, but they have not determined a clear motive, and her body still has not been found. His preliminary hearing is scheduled for February 19.

Sources tell CBS News that as part of her plea, Lee is expected to testify against Frazee, and possibly reveal what he told her about Berreth.

For Stein, she has "absolutely no doubt" that Lee was not involved in Berreth's murder.

Lee is not in custody, but faces up to three years in prison when she's sentenced.

CBS News has reached out to Lee's attorney but has not heard back.