SOS: how do i stop my dad from cheating?

my whole world is falling apart right now.. my mother just called me crying and told me about messages she saw on my father’s phone. a woman is texting him since almost a year now, it started all friendly and was about buying one of our cars but now she‘s saying the most DISGUSTING sexual things that make me literally so sick. she has asked to meet him numerous times but he always makes excuses and they haven‘t had sex or even a kiss (yet) but also my dad never said „no, i‘m married“. she is trying so hard to get him to have sex with her and has been taking her time doing so, i’m afraid it will eventually work. i can‘t sleep and can‘t stop crying, i am so heartbroken for my mother and our family. my mother doesn’t want to leave him, which i respect so i‘m thinking about how to make sure my dad never cheats on my amazing mother with this woman. i can‘t talk to him, i‘ve promised my mother and i can‘t betray her, what can i do?? also, this woman might be married as well- i just really despise her for trying so hard to ruin our family, she knows very well that he‘s married 😩😭 i‘m sorry for being such a terrible writer rn, but i‘m all over the place and need some advice. i can‘t tell anybody in my real life, i‘m 20 years old and i feel like i‘ve lost the man who i’ve always thought of as my hero.

ps: please don‘t comment anything hurtful or rude, i‘m going through a lot right now and am extremely sensitive to this topic, i really need serious and kind advice, thank you! :(