Please help me do the right thing here.?

My job at work is to install and maintain the computer systems. A fellow worker and friend ask me to go to her house and get a computer she bought on ebay working. When I got there, she was not home but her teenage daughter was. My guess is the daughter is about 16 years old. While I was working on the computer, the daughter’s boyfriend came by. After a while, I heard noises from a bedroom and it was oblivious the daughter and boyfriend were having sex. After I completed getting the computer running I had to walk by the bedroom to leave. They were still having sex and did not see me as I left. I know I am NEVER going back to that house.

One side of me says I should I just keep my mouth shut. What happened is none of my business. If I tell the mother, probably the daughter will deny she was having sex or say I wanted to have sex with her and then I am in big trouble.

The other side of me says that if something is not done I predict my coworker is going to be a grandmother a lot sooner than she wants to be. Should I tell her what happened? Is it possible that the mother knows what is going on and would be embarrassed and/or mad if I mentioned what I saw.

Moms, if this was your daughter, how would you want me to handle this? Please help me do the right thing here.