what to do when i catch my parent having an affair?

i am doing terrible...i just took a drive in my moms car only to find her cell phone left in it..i didn't mean to snoop in it but it kept ringing and I found all of these dirty text messages to another man (they were pretty sick). the worst thing about the situation is that the man...is my best friends dad and their family is pretty much our family (we spend all holidays together). i have no idea what to do. i could ruin two families if i say something. but its killing me. please no jokes/sarcasm here, this is pretty difficult for me.