I found out that my father is cheating on my mother right after I moved out of the country and now I don’t know what to do

TL;DR: I recently discovered romantic and sexual messages between my married father and other women. My parents have had a deeply unhappy marriage for years, and I’ve watched my mom cry countless times over how poorly he’s treated her. She’s also my best friend, and before I moved away we were together constantly. Now she’s alone with him and already struggling emotionally because I recently moved out. I’m devastated and don’t know what the right thing to do is.

I (21f) recently discovered messages between my father and another woman. He has always denied my mom or us access to his phone and I recently figured out his password and had five minutes to snoop. I found that he has been talking to many women on Facebook and recently arranged sexual and romantic dates with a few of them.
My parents have had a troubled marriage for years. They’ve slept in separate bedrooms for a long time and barely speak to each other, and my father has often treated my mother with very little kindness or respect. I’ve watched my mother cry countless times because she felt unloved and ignored and humiliated by the way he treats her. I know this because my mom is genuinely my best friend and, before I moved out a month ago, we were together almost all the time. She confided in me about everything especially how unhappy she was in her marriage. I was the person she cried to when she felt unappreciated, when she questioned why my dad treated her so coldly, and when she wondered what she had done wrong. As you can tell, finding those messages absolutely shattered my heart. myfather always presents himself as a very moral and religious person. He gives people advice about values and integrity, so seeing this side of him has completely shattered the image I had of him. I always suspected that he was cheating, and I’m pretty sure that my mom did too, but confirming it has been a huge shock for me. I also can’t stop thinking about the fact that she’s now alone with him. Moving away has already been incredibly hard on both of us because we were so attached to each other. She’s been struggling emotionally before and since I left, and I know she’s grieving the fact that I’m no longer at home. The thought that she’s dealing with all of that while unknowingly living with someone who may be betraying her is honestly breaking me. Part of me feels guilty for not being there with her anymore and I am seriously considering dropping out of this wonderful opportunity abroad to be with her and support her. She’s an incredibly sensitive person.

Any advice on how I should approach this would be greatly appreciated.