When Billy Ray learned of it, as well as at least one other fling, he filed for divorce Oct. 27, sources tell Us Weekly.
Michaels, 47, "became close to the entire family" this past February when he and Miley released the racy duet "Nothing to Lose," an insider tells Us Weekly.
He and Tish, 43, soon began carrying on an on-the-sly romance, sources tell Us Weekly.
Tish was seen at Michaels' Feb. 28 show at the Key Club in L.A., and he had once asked her production company's help to adapt his book, "Roses & Thorns," into a movie.
"Billy Ray was completely unaware of what was going on," the source says of the "Achy Breaky Heart" crooner, who is seeking joint custody of their three minor children, Miley (who turns 18 Nov. 23), Braison, 16, and Noah, 10. (They have three older children from previous relationships.)
Though a rep for Michaels refutes the allegations, telling Us Weekly, "There has never been an affair or a fling," and a rep for Tish also denies the dalliance, the Cyrus source insists, "It was a professional relationship that turned into something more."
RELATED ON WONDERWALL & MSN:
RELATED ON USMAGAZINE.COM: