AITA for not telling my friend her dad is cheating on her mom with her mom’s best friend?

So my friend “Sarah” and her parents and her dad’s girlfriend (he cheated on her mom with her) and her mom’s best friend are all going on a girls trip to Miami in March. I know her mom’s best friend and she goes to my church and I’m friends with her on Facebook.<br><br>She posted a photo from a few weeks ago of her and my friend’s dad at a football game together. So he’s cheating on Sarah with her mom’s best friend again. <br><br>I feel like I have to tell but my girlfriend and mom say I have no obligation to tell them cause it’s not my business.<br><br>But I feel bad keeping this secret.