My mother has an extramarital affair

so I just saw some DMs on my moms instagram and when I went through it, I saw that she has been texting this random ass guy since a month now, they're flirting back and forth, and she's sent him her selfies once or twice.
Now you have to understand that way before, my mother's instagram bio used to be a typical millennial bio, "coffee lover, mommy of two lovely kids" etc, however that has been changed and she no longer displays any sign of having kids or a husband.
My parents have been married for 18 years now, we are very well off and I don't get why she would be needing another man? my father has given her more than she's ever asked for, he doesn't abuse us in any sort of way so like I'm clueless what has driven a grown woman to do this. Now I just need some advice on what to do, I was thinking of catfishing the guy himself but I don't know what good that's gonna do for me.
I would tell my father about it but I feel like that's going to cause more issues, my brother has autism and he's very attached to our mom, and even if they do get a divorce, my dad would get custody of us both because my mom does not have any qualifications and cannot earn a single penny on her own.