Found out my has been cheating on my mum. What do I do please?

In September 2022, when I was moving into my uni house for my second year of university, I had to log into my email to access something for my phone due to mine being broken. As I was about to log in, an email account came up which I had never seen before and a message from a woman who me and my mum call 'the crazy woman' as she always would call my dad with her problems during the lockdown and etc (they worked together). The emails were very suspicious but not enough for me to think something but from then on I had a feeling. One email on the 7th may 2022 from the woman saying 'I told you a million times how I feel about you' 'I am more invested in this than you are' 'I dont know how long I can be in this subordinate position and follow your lead when you want to talk to me' and also that something had culminated several times. My dad did not reply to this email and it was really the only email out of everything that implied something was going on but due to him not replying I just thought she was living up the name of 'crazy woman'

I asked my mum if she had met her and my mum said my mum and dad went out for dinner with her and her husband. I then put it to rest as I just genuinely didn't want to think this could happen. Small things happened then like my dad saying he wants to try taking my mum on holiday etc so I just thought maybe its nothing. There was also another email sent to my dad of her now husband telling him plans for there wedding.

A few weeks ago me and my mum came back from a concert and while my dad picked us up, the woman called my dad in the car. My mum annoyingly groaned and my dad quickly declined it. Something in me was just telling me this isn't right.

I then asked my dad that his friendship with this woman was weird and he pulled a face and said is it and said people come to me for advice, they always have then proceeded to talk about work. I left it after that but still had my suspicions.

Yesterday, my dad left his phone in the toilet and as bad as it is and know some people will say don't go on peoples phones and I really was not going to but I went on and saw messages between my dad and this woman. They were inappropiate and it is clear that he has been cheating. From the messages I saw, this is no misunderstanding. My dad is 61 and my mum is 54 and have been together for over 22 years. They are not married. This woman is 29!!! and married and even posted a couple weeks ago on her Facebook happy anniversary to her husband. I am genuinely in shock and I dont know what to do. I cannot stop crying, I cant eat and I cant sleep. I have no siblings I can talk to about this at all as I am an only child. My heart breaks for my mum and she would in no way be able to be dependant without my dad. I am confused as it was my mums birthday a couple weeks ago and he bought her a card saying 'to the one I love' and surprised her with all these lovely things. They share the same friendship group. They have holidays upcoming. I am ultimately scared what this will do. I have had the 'perfect' childhood growing up with them. I am now 20. Family holidays, christmas, birthdays. It has all been perfect. I dont know any different and I am just unsure what to do. I am at uni for half the year and will not be able to be there for my mum if this all comes out. I still love my dad but I cannot believe he has done this to us. I also feel for the womans poor husband. My heart breaks for my mum. What do I do?