This issue could possibly be more important to you than to him. He might think the Only Fans account is an insignificant lark, something either fun or profitable that doesn't matter much to his friendships and other relationships.

Me, I'd probably let it somehow slip that I knew? I wouldn't make a big deal of it, I'd probably just make a little reference in conversation, with context that made it under good that I wasn't judgmental or anything.

Friendships are strongest when they're open and transparent. That's how I feel, anyway.