Venise so, her dad has turned out not to be a perfect person, this is actually a realistic view of what people are like, including our parents. Of course it is nicer if we have loving kind parents who create a welcoming family home and stay faithful for nearly 40 years but the reality of this is very far from the case for probably most of the population.
I think there comes a time where you have to decide if you can have a relationship with your parents as they are, not as they would ideally like to be. My dad has had multiple affairs (no longer with my mum) but they are not my concern, they are his partner's concern and I just don't get involved in the love lives of my parents, both of them have made daft choices over the years, especially when they went back into dating/seeing new partners.
I think it is harder when they are in a long marriage and are still at the core of family life. I would think that the mum in this situation does know as his behaviour is so blatant that the son is questioning it, but for whatever reason she is not proceeding with asking more about it.
But- sadly the mum's tears are not going to dry if she finds out her husband is unfaithful, nor is she going to necessarily be happier starting again aged 70.
I would keep out of it, given there's enough signs there for the mum to ask her own questions, she must have considered what is going on but is too afraid to ask.
I don't agree that when you have adult children, having an affair is 'betraying the whole family'- parents are grown-ups and grown-ups sometimes have complicated lives.