Dad cheating on mum HELP!

I posted here yesterday about this situation but in short, I found messages between my dad and his 25 year old coworker, it was clearly sexual, as suggested I confronted my dad, he laughed at me and said it was only friendly banter and it wasn’t like that but the messages didn’t lie as there were also nudes involved. I then messaged the 25 year old and she proceeded to not only block me without explanation or reply but also my best friend. I now have no idea what to do, I want to tell my mum but she is an alcoholic and very unstable I suspect because she feels like something is going on but she has been an alcoholic for years, she deserves to know I know that but I want more evidence asides from the clear messages between them two, or at least some support while telling her, please help any advice is appreciated