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."