"I thought I'd never sing again": Shania Twain on her friend's affair with her husband.

Image via Instagram (@shaniatwain).

It’s an unwritten rule that friend’s exes are totally off limits – and of course even more so when they’re your current partner.

So what do you do when you find out your best friend and husband are having an affair? If you’re Shania Twain, you marry HER ex. Obviously.

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While the real-life wife swap may sound like something straight out of Days of Our Lives, the 49 year-old country singer recently revealed to the New York Post just how heartbreaking finding out her husband (and manager)  Robert Lange was involved with her best friend, Marie-Anne Thiebaud in 2008.

“When that happened, I though, ‘Forget it, this is more than I can handle – I’m never going to sing again’. I had to grieve through it,” she told the Post. (Post continues after gallery.)

The pair had been married since 1993 and have one son, Eja born in 2001.

In her 2011 memoir From This Moment On, Twain revealed that it really knocked the multi Grammy award winner’s confidence.

“I lost my sense of trust, honesty and compassion. I crashed down and became what I consider an emotional mess. I’ve never been so miserable in my whole life. I just wanted to go back to bed and never get up,” she told Oprah in 2011.

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She also revealed that both have never admitted to any infidelity.

“I never did get any of the details. I phoned Marie-Anne, of course, and she changed her number and that was it,” she said.

The affair really shook Twain. Image via Instagram (@shaniatwain)

Twain believes the trauma caused her voice to "shut down" and she became convinced she "would never sing again." A doctor diagnosed her with dysphonia, a condition that causes the muscles to tighten.

However the person responsible for the ultimate female anthems like "Man I Feel Like A Woman" and "That Don't Impress Me Much" (yes, we've all drunkenly AND sober-ly sang along to them) soon picked herself up.

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She found love with Thiébaud's ex husband (yes, really), Swiss-born Frédéric Thiébaud, whom she married in Puerto Rico in 2011 and is currently embarking on a final tour, #RockThisCountry.

"It was twisted," she admitted to Oprah, "But so beautifully twisted. I really can't complain. I've got a beautiful man."

What's even more surprising is that she's apparently cordial with her ex - for the sake of now 13 year-old Eja. "I do see Mutt through our child and that goes very well, it honestly does," she said in the same interview.

However the same can't be said for his wife and her ex-best friend. (Post continues after gallery.)

"I don't see her, ever. I don't invite that trigger into my life... She's not my future - she's my past," she told the Post.

In an interview with US TV host Andy Cohen, Twain recently said she wishes she'd "never met" her, and when she ran into her a few months ago, she had a "total panic attack."

"I just told her that she was a bad person - that's all I could get out!", she said.

RELATED: Mia Freedman: “Life’s too short for toxic friends. This is how I got rid of mine.”

Unfortunately, Twain isn't the only celebrity to experience the betrayal of a supposed best friend - earlier this year, former Masterchef contestant and current TV Chef Poh Ling Yeow revealed her ex-husband Matt Phipps also married her best friend.

While she told the Daily Mail it was a "World War One" breakup, the 42 year-old says, surprisingly, the trio remain good friends - and Phipps is still her manager.

"It wasn't a shock, I had a feeling they had a connection. There was no need for them to ask for my approval. I was ready to let go," she said of learning about their relationship.

"There was actually, very strangely no transitional period."

Ling Yeow is now happily remarried to Jono Bennett, who she met on the production crew whilst filming Masterchef.

Have you ever found yourself in this situation? Could you remain friends with an ex or your best friend?