Found out my step dad is cheating on my mum

[2025mj]

Found out tonight that my mum seen very explicit messages from a girl that he works with. No evidence that anything physical has happened but the messages are of a sexual graphic nature. They also phone each other multiple times a day.

I used to work in the same place as them and I'm livid. I know it's not my place but I just want to scream at her.
She's even younger than me.
I am distraught for my mum and I don't really know what to say or do right now

how awful. Just be there for your mum x

Sorry I know aibu isn't the right place and im really not in the right head space for a pile on, I just needed someone to talk to. They've been married for five years
How and why I don't get it

Thank you. She hasn't confronted him as we have a huge family party to attend tomorrow
And it's her birthday the following weekend.
My mum is such a good person, she didn't deserve this

If I were you, I'd want to scream at him, not the silly young woman who's been taken in by him. He's the one who cheated on your mum. I'd never be able to be civil to him again. I hope your mum isn't going to stay with him.