Annoyed. P.S long!

Chelsea • 25 years old! Mommy to an angel baby, and a beautiful little boy! 😘
So I recently found out that my dad is cheating on my mother. This is not the first time he has done this. It ALL started over 13 years ago, and has been an off and on thing. I'm 22 years old, and I work full time with my dad. I have found multiple text messages in his phone from this women about how her dog is 'getting more' than he is. (I would just like to say, I was not snooping through his phone, mine was not working correctly and I brought up the texts to message my boyfriend and that was what came up right away) He's brought her to MY house, and he's been to hers many times. I've walked in on them and anytime I confront him, I get 'we're just friends.' I have told my mom every time that this happens, she hasn't done anything because she has come to terms that he's the one who has to live with himself and he's the one that's giving up on the relationship, she feels that if he's really that unhappy, he needs to be the one to file for docorce. Not her. 
I'm torn about sending her a message because I feel that no matter what I say, they're going to do as they please anyway. 
I guess my questions at this point would be, 
1: would you continue to work for your dad, or would you get a different job?
And
2: I have the other woman's number... Would you do anything with it? Or would you just leave it alone?