I agree with you both on princple... why should anyone surrender ground when they are the one's being hurt by and large... ?
BUT when surrendering that ground reveals to the WS the damage they are doing it can be quite productive...
Legally there may be cause for concern, so look into that.
I would note that any legal pursuit should be noted to the WS to keep things OPEN and fully disclosed.
The note I would attach is this :
I am taking steps to protect our marriage and our family from the OW and your affair. I am seeking a lawyer's advice on matterns concerning protecting our children's home from the OW and the harm she is doing. You are entitled to know I am seeking an attourney's help to protect our marriage and our family.
This makes it clear what her objective is, ALWAYS ALWAYS ALWAYS set a powerful example of adulthood whenever you make a choice like this in hopes the WS catches on and follows suit.
This keeps the WS from filing for divorce and jumping to the conclusion that his marriage is over.