I don't know what's the next steps

I'm 21M, my friend (let's just call him Jack 28M) told me his sister (Alicia 16F) has a crush on him as well as he has a crush on her. He told me a little about how they're attracted to each other. I just wanted to tell him that he could tell me anything, I'm not judging him. I told him that if anyone found out how it would be really bad. He told me as a friend he can trust that he wants to be intimate with her. I'm really conflicted as to what to do now. As a friend I want to be able to support him, but as a human I do not want to be complicit in something like this. I'm really stuck on this and I'm not sure what to do next. Any advice?