Gayle King Recalls Finding Out Ex-Husband Was Cheating With Her Close Friend

Gayle King is reliving one of the most brutal betrayals of her life -- catching ex-husband William Bumpus cheating ... with her own best friend.

Appearing on the "Call Her Daddy" podcast on Wednesday, Gayle said she got back from the airport earlier than expected one day, and immediately knew something was off when the house alarm was set -- something William apparently never did.

Gayle says William bolted out of the bedroom, wrapped in a towel, and tried to block her from going in -- warning her that someone was inside.

She said she searched the house and found a woman hiding behind the door ... also wrapped in a towel. Then came the gut punch -- the woman was her best friend!

The "CBS Mornings" host says she asked how she could do this to her ... but the real kicker? Gayle says William then drove the woman to the train station so she could get back to her own husband.

Gayle later called Oprah to vent ... and Oprah apparently told her that the incident suggested the betrayal ran way deeper than she realized.