Same Gender Abused

House please help. A friend of mine confided in me about what transpired between him and a man he has been close to all these years and looks up to.

 They had an event that ended late at night, and they had to pass the night in the same room, and same bed. He said at night, he felt someone touching him in the crotch area. He felt it wasn't real, but it happened several times again. He said he wanted to protest, but something told him to keep still. 

He also mentioned that something similar happened a while back with the same person, he confronted the man in the morning, and the man said it was his imagination, that nothing of such happened. He dismissed it, as he was very exhausted the previous night and couldn't remember so much. 

Now he's sure this last incident happened and now he feels bad he didn't protest the night he happened as the man seems to carry on as though nothing happened. He looked up to this person, and they are in the same social circle. He doesn't know how to detach from this person without raising concern from people around as they were pretty close and he doesn't plan on making his discovery public. He said he wants to have a "litmus test", hoping the man might make advances at him again, and he'll actually "catch" the man in the act this time.

 That way, the man would be aware he knows, and detaching from him wouldn't be difficult. I however told him I do not think it's a great idea.

please what do you think he should do