My dad is having an affair?

Ok, so my mum and dad split many years ago, I live with my mother about 200 odd miles from my dad, I try to see him as much as possible, my dad remarried when I was young, I hate his wife always have always will, but now I think he's cheating on her! As much as I hate the woman I still think it's wrong for him to do this :/ and just recently the woman he's having an affair with is messaging me on FB! What should I do??? :/ thanks for any help :)