Shania Twain's private life has always been just that -- private -- but the meltdown of her 14-year marriage to music producer Robert "Mutt" Lange has unfortunately forced her into the spotlight.

People magazine reports the low-key Canadian singer, 42, "is reeling" over rumors that her estranged husband, 59, with whom she has a 6-year-old son, Eja, is involved with a woman "she considered a close friend."

Insiders contend that Lange's ostensible closeness with one Marie-Anne Thiébaud, a secretary and house manager at the couple's Swiss chateau, led to the split.

"Mutt and Marie-Anne left their spouses for each other and are still in a relationship," a snitch tells the magazine. "Their two families would vacation and spend holidays together. Shania considered Marie-Anne one of her best friends."

Shania, who is reportedly being comforted by friends and family in her native Canada, "is devastated" by the breakup, says the source, who adds, "This came out of left field."

Another spy echoes the suddenness of the separation to the New York Daily News, asserting, "It took Shania by surprise. The divorce could get ugly. Shania may want custody of their son and alimony."

Lange and Thiébaud, however, are denying the two-timing claims.

The camera-avoiding producer tells People that his union didn't implode over something so clichéd as an affair with his assistant, insisting infidelity was "absolutely not the reason [for the breakup] ... It's literally just a growing apart, that's all."

As for the talk that he's seeing Thiébaud, Lange maintains, "It's not true."

Thiébaud is also defending herself against the other-woman charges.

"I know what you're ringing to ask me about," she said when reached by E! News at her home in Geneva, Switzerland. "But, when marriages break up they break up for all sorts of complicated reasons, it's never just one reason. And those reasons should be kept private."

Thiébaud, 37, who says she no longer works for the reclusive couple ("I am retired"), then suggests the media should bother someone else.

"If you're looking for a person that broke up a marriage," she huffs, "then you have the wrong person. I'm not that person."

Next: In Defense of Scarlett's Rock, Reputation