When she gives him what he wants (a clear end to their old romantic R -- she really has to let go and not remain in the old M by herself). Once he is left with his self-loathing, he will figure out that her past is not the cause.
It will likely make him very angry. WASs spouses think everything will be made better by getting rid of LBS, but this doesn't fix things. The WAS will likely even be mad at LBS because leaving LBS didn't fix things.
Mamabear and her H are not in a place where it is appropriate to try to work things out. The sexual history issue is one that needs to be resolved between H and W. They are not effectively H and W. No amount of rational discussion will persuade her H that she is not the source of his unhappiness, that abandoning a real M with her is not the magic pill to cure him. She has to let him try it for himself to see that it doesn't work.
Of course, when he figures out what does work, it may or may not be compatible with an M with Mamabear.
