What should I do about my son"?

I am hoping to get some insights into this situation because I don't have people to discuss this with. My son is 18, recently he has been leaving our house and not telling me where he's going. At first it was just sometimes but then it began more often. I was worried so I decided to go into his room and check all of his stuff including his laptop. I managed to find messages between him and an older man. The man is my friends parent and they are having sex from 3 days after he turned 18. The man is 56 and it grosses me out. I confronted my son with the evidence and he confessed the man invited him home and seduced him, but my son says he wanted it and is fine with it. I told him he is being used and that the older man is a pervert. He got mad that i went through his stuff and that i want to ruin his fun

I went to my friend and told her what her dad is up to. Shockingly, she thinks since they are both adults, i should let them be. I told her she should be ashamed of herself and her father and distanced myself from her. At this point, what is there to do?
What should I do about my son"?
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