Similar but not exact situation. My thoughts, worth exactly nothing, but I can suggest suggestions from counselor and being old and weathered.
Kids are in the process of separating, which is normal, but not easy for any involved.
You can't fix it. Step back. Be kind but unattached. Treat them as you would the checker at HEB, kind but not offering suggestions or help...unless requested, and then tread lightly. As I am sure you know, they don't want your suggestions. They see it as you saying they don't already know this stuff. They see it as infantilizing. And in fact, they do already know. Trust your previous parenting. Play the long game. All easier said than done, especially for dads who have the natural instinct to fix it.
I call it "Be the Queen" patterned after the Queens on The Crown. Kind. Interested. But not offering solutions.
Trust your previous parenting. You know you did a good job.
As for the spouse...I got nothing.