I don't know what's going to happen because my mom said if she gets legit evidence that he's cheating, then they'll divorce.
If only that was how divorce actually worked..
Divorce usually requires mutual consent. If your dad doesn't want to divorce your mom, he can sleep with as many girls as he likes and still not sign the divorce papers. There's nothing your mom can do, she can't force him to sign the papers and without his consent divorce cannot be done at that time.
The only time where divorce can be done without mutual consent is if both parties have been separated for more than two years and a court finds that there is no plausible way the marriage could be mended.