People Exclusive

First Photo of Alleged Other Woman in Shania Twain Split

By Alexis Chiu

05/22/2008 at 06:15 AM EDT

As Shania Twain copes with her sudden split with the support of family and friends in her native Canada, sources close to the singer say she is reeling from a double betrayal – not only by her husband of 14 years, music producer Robert "Mutt" Lange, but by a woman she considered a close friend.

Lange's relationship with Marie-Anne Thiébaud, 37, a longtime secretary and house manager at Twain and Lange's estate in Switzerland, was behind the breakup, say several sources familiar with the situation.

"Mutt and Marie-Anne left their spouses for each other and are still in a relationship," says one source, adding that the Swiss employee was a fixture in the household Twain, 42, and Lange, 59, shared with their son, 6-year-old Eja D’Angelo.

"Their two families would vacation and spend holidays together," says the source. "Shania considered Marie-Anne one of her best friends."

After the separation was announced, Lange told PEOPLE exclusively that an affair was "absolutely not the reason [for the breakup]... It's literally just a growing apart, that's all." He also denied that he was in a relationship with Thiébaud, saying, "It's not true."

Whatever the reason, Twain "is devastated," says the insider. "This came out of left field."

Like this story? Click on the Yahoo Buzz! button below to Buzz it up!