my best friend is getting groomed and idfk what to do

my best friend (16m) is knowingly and willingly in a discord relationship with a 27m who's using him for sexual purposes, I told my friend multiple times that he's getting used sexually and that this is dangerous but he knows all of it and he doesn't care cause he's been groomed in the past so now he only goes for older people. It's a trauma response you could say. He doesn't even like the 27 year old that much romantically but he's staying because he wants to live in his basement when he grows older and be his sex slave. Cause he gets a free house. They've made plans for it already and not to mention my friend isn't the only one getting groomed by this guy. He'd in a polyamorous relationship with him and a 15 y/o so this guy basically has a brothel planned. Again not to mention he said the lowest he'd go is 13 years old.

i genuinely wanna help my friend but he doesn't care. But i wanna somehow report this guy behind my friend's back. I have two photos of his face but nothing else. And a few screenshots of what he's been saying to my friends but that's it