<human>: Hello, everyone. This is my first time posting to this sub, so feel free to tell me if I do something wrong.

I put 'might be' in the title as I'm still not completely sure. However, I found some suspicious things.

First off, a few weeks ago, I borrowed my mom's phone to check the weather. When I opened it, I saw a message from someone with a guy's name (in the diminutive form, which, in my language, is something only for close friends or SOs). It said something like 'How are you doing, love?' Once I saw it I immediately put it down and didn't read further. I now realise I probably should've at least checked their other messages, but I don't really want to know any more.

Another thing that made me really suspicious is, when my mom recently asked me to go on her facebook, and her password was the same guy's name.  

For context, my parents never had a loving relationship, or any relationship at all. I don't remember a time they talked about something that didn't concern my siblings and me, or anything they didn't *have* to talk about. They are great parents, but they are more like roomates who yell at each other sometimes than a married couple.
  
Now, I'm about 99% sure that my mom is cheating on my dad. Of course, he could be just a friend, but I really, really doubt it. Anyways. my question is, should I tell my dad? It is the right thing to do, but I am afraid they will have a divorce. My uncle and aunt divorced a long time ago, and it was rough for my cousin - and still is - dealing with the anger and at whose house he should stay all the time. On the other hand, my dad is a good person and doesn't deserve to be cheated on.

Please advise me on what to do here, Reddit. I'm feeling guilty that I'm helping my mom cheat in a way, so I would really appreciate your help. 

Sorry for any grammar mistakes, English isn't my first language.

Can you give me a shortened version of the text?
<bot>: I have reason to believe my mom is cheating on my dad, but I'm not sure whether to tell him as I don't want them to divorce. What should I do?