First you gotta determine what you believe your mom would do if she found out (and trust me, she will find out). She gonna flip out, get violent, ask for divorce right away, cry, try to reconcile, etc.
Next you gotta ask if you're ready to deal with the short term blowback that will occur if you expose him. Your parents could get divorced, your family could be split, etc. Or if they don't split, they'll always have problems - your mom won't trust him, may take some of her anger out on you, your dad will be disappointed that you outed him, etc.
Without going into specifics I ran across evidence of my dad cheating, but wasn't sure. One time we were hanging out at a sports bar. I got up to take a leak, and when I came back I noticed some chick was in my seat talking to my pops. I casually walked over there, and could tell something was going on. She got up, said hi to me, and left. My dad whipped out some bullshyt like "oh that was just a coworker..." but I could tell something was up. I didn't say anything to my mom.
Few years later my mom found out and went nuts. She wound up getting arrested for assault due to attacking him, but he dropped the charges.
So breh remember, whatever happens she's gonna find out. But if you tell her, be ready for the consequences. Maybe a better option is to leave the evidence out in the open on the computer for her to see it. That way you don't have to directly tell her, she'll see it herself.