Would this be considered cheating?

I accidentaly came across a live s*x chat site that my father has been using. Its where you get on webcam and actually, do *things*. I dont know if I should tell my mother.. Should I? Would you be mad if it was *your* husband? Chatting on a live s*x site with other women.. ?? I dont know how I feel about it. I would never be comfortable talking to my dad himself about this. I just dont want him doing things that my mom wouldnt want him to do. This isnt a normal porn site. Its like if you were on skype. its a live webcam chat porn site thing.. what do i do?! and i dont want them to get a divorce. i dont want my dad to be mad at me.