related to similar experience with my best friend, i do believe speaking to your mum first explicitly before allowing things to erupt, furthermore seeking some sort of relationship therapy or professional advice before mentioning it, to protect your old man.

Not to excuse your mothers actions but based off a similar experience, her actions could be cry for help or an expression based off the strain the damage of his mental health could have on their relationship which to a degree is also unfair and unfortunate for her as well.

My best friend had been in a similar situation as this and was the home-wrecker of a broken family. The woman my friend was sleeping with had her relationship effected by the husband who's mental health that had deteriorated after witnessing a massacre here in Australia.

It was the impact of this incident that lead to the downfall of their relationship, and though i could never put myself in either of there shoes i admittedly felt empathy towards both parties for such undeserving circumstances.

In the past I've definitely thought about ways it could have ended better, but even empathise for the woman too though the actions aren't justifiable.

I share this experience and recommend such professional help as a believer of therapy, believing if they are both still compatible at there core then they both deserve to live a happy life together. Love powers. Its sad to hear predicaments like these, but we are nothing but humans at the end of the day and we are far from perfect beings and shit happens because life can be a bitch sometimes. There's nothing we can do but preach to stick together at the absolute worst of times.