eddiecat78 "When she is like this he gets glimmers of hope that she can change and they can live happily together and he feels that he should stay for the sake of the children. However, he is rapidly reaching the point where he feels that the occasional good day does not make up for the many many bad days." Who tells you these things? Is your son violating the privacy of his marriage and talking to his mum, complaining about his wife to mum? He's surely not putting his wife above all others is he? United front against the world and all, I guess he forgot the vows.
If you have problems in a relationship, should you not talk about them to a third party? Speaking from experience, if you're trying to maintain a relationship with a 'toxic' person, it's very isolating. You doubt yourself because the toxic behaviour is often inconsistent, and if you raise the issues with the person concerned, my experience is that they often deny/minimise the problem (or stop speaking to you entirely). I nearly lost my marbles trying to keep my issues with my mum to myself! It was only talking about it to others which got me to a place where I could manage my mum's behaviour (to an extent), accept the situation, and avoid triggering issues while still being 'myself' i.e. having my own opinions and parenting in the way my husband and I think best - which is how our relationship has survived and hasn't resulted in estrangement.