Heather Elvis' Friend Speaks Out: "I Don't Know Why They Did It"

When Heather Elvis started dating Sidney Moorer, her friends were not pleased.

She was a 20-year-old waitress at a pub called the Tilted Kilt, a Celtic-themed bar and grill in Myrtle Beach, South Carolina, and he was a 38-year-old married maintenance worker employed at the same bar.

"Heather was a very pretty young girl, and then you have this scruffy guy with his hair in a ponytail," Elvis' best friend Brianna Kulzer, 28, tells PEOPLE. "I was like, 'Heather, are you sure? Are you sure that's what you want to go after?'"

heather-elvis — Heather Elvis. Courtesy Find Heather Elvis

Elvis reassured her friend that the father of three had an open marriage and his wife, Tammy Moorer, had a boyfriend.

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"She was under the assumption that he was going to leave his wife and that they were going to be together, and he loved her and he wanted the same thing," says Kulzer. "l looked at it as something that could potentially blow up."

It did.

Heather Elvis case in Conway, S.C. — Sidney Moorer. Janet Blackmon Morgan/The Sun News/MCT/Sipa

According to prosecutors, Tammy learned about the illicit relationship in October 2013.

"The wife threatened Heather and called her over 60 times one day, when Heather was at work," says Kulzer. "She told Heather, 'If you don't stop talking to my husband, I'm going to come after you. You're going to have problems with me.' That's pretty much when it ended."

Kulzer says a shattered Elvis later called her friend crying.

"She said, 'Sidney just called me,'" Kulzer says. "'He said he left his wife and that he wants to see me. He's sorry for what he said and he wants to be with me.' I said, 'Sleep on this and we will talk about it tomorrow.'"

Kulzer never heard from her friend again.

The Heather Elvis case is explored in the new episode of Disappeared in the Darkness: An ID Mystery, premiering tonight at 9 p.m. ET on ID. A clip is above.

Prosecutors claimed Heather drove to the Horry County boat landing in Myrtle Beach — where prosecutors said the Moorers lured Heather before kidnapping her.

Heather's abandoned car was discovered near the boat landing the following day.

Two months later, the Moorers were taken into custody.

Tammy Moorer was sentenced in October 2018 to 30 years in prison for kidnapping and conspiracy in Heather's disappearance. In September 2019, Sidney Moorer was sentenced to 30 years in prison for kidnapping and conspiracy to kidnap.

Heather has never been found.

For more on Heather Elvis's disappearance, tune in to Disappeared in the Darkness: An ID Mystery, premiering Wednesday, May 27 at 9 p.m. ET

Updated by Christine Pelisek