I doubt he will be blindsided. He must know his marriage is on the rocks.
Just be happy there are no children involved.
No he had no clue. They had some problems 6 months ago but he thought they worked past them; apparently not. When he came home and saw the note, my other SIL said he almost collapsed, and he’s 6’5.” He’s devastated and lost, but he never said a bad word against her to anyone tonight. He’s a gem and she’s gonna regret this decision. I hope they can work things out, but they definitely need marriage counseling to learn how to communicate better.