I Caught My Friend's Dad Cheating On His Wife! What Do I Do?

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This past weekend I walked in on my friend's dad cheating on his wife with my friend's girlfriend. Mario has been one of my good friends for about two and a half years now, he is twenty three years old. He has been going out with his girlfriend Heidi for one year now and is very in love with her. This Saturday Mario asked me to meet him @ his house at 2 pm since his soccer game ended at 1:30pm. I left earlier to grab some food on the way but I over estimated the time of arrival and got there about 15 minutes earlier than I was supposed to. On the phone Mario had told me that Heidi would be there as well and if nobody answered the door to just walk in, since his family used to me being around this is ok to do sometimes, I noticed Heidi's car was parked on the street and thought she would let me in but after I rang the doorbell 4 or 5 times I went inside the house. On the way to Mario bedroom is the door to his parent's bedroom where I found Heidi and Mario's dad...well you know. Neither saw as I just left the house and waited in my car, I pretended like nothing had happened the rest of the day. Obviously I can't get this out of my head and I just want to know what I should do about it, who deserves to know and how to handle this dilemma, thanks in advance!