I just found out that my mom is cheating on my dad. What should I do?

[Genni]

Lately my mom has been "working" very late. She leaves around eight in the morning and doesn't come home for twelve hours at times when she is only working eight hours. My sister (20) thought that maybe my mom was with this guy friend of hers that she used to do things with a lot, so I (21) looked at the data usage for her cell phone and she hasn't had any contact with him, but there was this other number on there whom she has been talking to quite often and for long periods of time. So I went onto her computer because she never logs out of Facebook and noticed all these notifications from Zoosk, a dating website. So I went on there into her inbox and discovered that she has been talking to this man (who is nine years younger than her) for three weeks and has been seeing him for at least two. They're also having lunch together today. And his phone number matches the one I saw on the data usage. She has told this man so many lies that I am sick to my stomach over it. The most hurtful thing was she told him that she's widowed and that my dad died four years ago from a brain tumor and that my sister and I were pushing her to date again. I don't know if my mom and this guy have had sex, but they've definitely done other things. I just don't know what to do and I am so hurt. I knew my mom was a compulsive liar (she lied about having cancer twice), but this just takes the cake. I want to tell my dad because he doesn't deserve this. He is the nicest person and basically worships the ground my mom walks. I know that he loves her so much and she just takes him for granted. I don't understand what she is thinking. She has had so many friends of hers get cheated on by their husbands and has always said, "If you want to cheat, then leave the marriage first." My parents have been married for 22 years now and I don't think this is the first time she has cheated on him. I can't believe my mom would do this to my dad. I know I need to tell him, but I don't know how to go about it. I know that this will crush him and my mom will probably want to disown me. I really want to call my Grandma (my dad's mom) for advice because she knows how my mom is, but at the same time I'm not sure I want to tell her before my dad even knows. I haven't even told my sister yet. I just don't know what to do.