What a dreadful position to be in, whatever you decide to do will have repercussions.
If it was me, I would have a chat with my mum along the lines of ..have you noticed this that and the other about dads behaviour recently, and do you think he is maybe up to no good again.
She will soon figure out for herself if I know the average scorned woman if he is having an affair, but it will also give her a little time to get her head around it and plan what to do, I hope it all works out for the best but must be a worrying time for you.