My dad has had two affairs with two different women (that I know of) and both times I was the one who found out and told my mum, which was very difficult. She chose to stay with him both times, I think out of convenience, as I know they're both unhappy. First time was around 2018, second was 2021. Both times my dad said he broke it off. This brings me to present day: I noticed that my dad and the first OW are friends on social media and are liking each others pictures as recently as the last few days. She has also been keeping tabs on me as I can see she's viewed my stories on social media from the last few days too even though we aren't friends (my story was inadvertently set to public instead of just friends, but I decided to keep it this way just to see how much she's looking at my profile).
My question is: given that neither time I've told my mum she's done anything with the info, should I not tell her what's going on this time? Even if he's not seeing her now, why is he friends on social media with the woman he had an affair with? I'm extremely close with my mum, not close with my dad so my loyalty is definitely to her, but would I be helping her by telling her or not? I'm also feeling fed up with my dad putting me in this position, but hate to think of my mum being in the dark. As adult DC to our parents, at what point do we just mind our business?