Okay, LW IMO your W sees your S as a way for you to continue to control her.

Somewhere along the way, W decides she is not going to take it anymore. She is self righteously going to take a stand. Make some changes for her.
Whatever "it: is. When S is dragged into this scenario she as others have said see it as a threat to this commitment to make these changes.

Buddy, leave it alone. She will either permanently screw up her relationship with her S or not. S will be fine.

M 53 W 54, M since 98
D15, D19
8/2013 discovered EA, W maintained contact with OM
until 10/14
7/2/17 W said she wanted S, 7/25/17 moved out of MBR
12/17 W says moving out 5/18, W still in home.