I really like @RichardU's answer, but I have a bit more that I'd like to add so I decided to open another that may provide another lens.
First, full disclosure, I've been on all sides of this issue. I've been a referred candidate, I've referred others when I wasn't part of the interview team, I've referred others when I was part of the interview team, I've been hiring manager when folks in my department (and on the hiring team) referred people, and I've even referred others when I was the hiring manager (to my own department even). I've successfully referred something on the order of 10-12 folks over the years, and unsuccessfully referred a number of others. I'll go through my approach to each and perhaps it will be useful, perhaps not.
As a referred candidate my friend spoke on my behalf to the hiring manager, got me the interview. His only advice was to tell me some of the company values at the place I was interviewing. He didn't give me much insight into the hiring manager or other interviewers (though to be fair, I don't think he knew them well and wasn't part of the interview team). The only specific thing he said in that regard was that my phone screener had a soft spot for folks that had a particular job experience that I had.
when referring others
In cases where I wasn't on the team, I did exactly what my friend did. I talked about the corporate values, I sent the candidate the job requisition (in case they didn't have it) and I let them know what I knew about the department they were interviewing for at a general level. I gave them anecdotes of my own hiring experience as well.
When referring others as a member of the hiring team
In this case, I recused myself of participating, I made it clear to my manager that the candidate was my friend. I did give my friend a general framework for how the interview would proceed (i.e. what steps were involved) and I did tell him what the department culture was like. It was a software department, and so I made general advice like, "You'll have a technical interview, they will probably want to test your understanding of computer science algorithms" and "You have a technical presentation you have to give. A lot of folks attend, you'll want to strike a balance between story telling and skills demonstration." I did not specifically tell him questions and I did not tell him specifically what attributes the team was looking for. I constrained myself to the advice I would have given had my friend been interviewing for a software position in any company. Important to note here that despite recusing myself, my boss did ask me to join the decision meeting, where he asked me to explain to everyone what things my friend had done and what skills he had that had caused me to want to refer him. This was actually really hard for me because it turns out my friend had not performed well on some parts, and so I had to listen to the feedback that wasn't so favorable. They ended up not hiring him.
When hiring referred candidates
Frankly, regardless of prep there are two things that happen here. 1) I assume the referrer does tell them all they can about the department and the interview process. Frankly, I don't think it matters that much. Our process isn't designed to favor folks who studied the test, it's designed to give us a good sense of how they think. We have a specific block where we give them a question and a half hour to do research and design and we don't require them to finish, just to explain their thought process. 2) I (like my boss in the previous block) ask them to talk about the candidate to us, explain things that we might've missed in the interview, etc. It's like free info, and while it doesn't add weight by itself it's good context.
When referring as a hiring manager
This one was the trickiest, but I feel like I did the right thing. In this case, I made the referral, formally recused myself of the bonus, and then handed all decision-making power to my assistant manager. He's supposed to be able to step in when I'm gone anyway, and I have worked with him long enough to trust him. I handled the candidate exactly how I did above (as even as the hiring manager, I consider myself part of the hiring team, and generally defer to the group's vote on hiring decisions). I explicitly told my assistant manager that the decision was his and that I didn't want my friendship of the candidate to have any bearing. I am aware that it would anyway, but I wanted him to know that I really did want him to decide in either direction. As it turned out, my friend did very badly. It's a long story, but he upshot is that he was having one of the worst months of his life (multiple medical, personal, and family issues all at once) and tanked the interview. I knew it just talking to him afterward, and when my assistant manager called me later, he was expressing uncertainty about the candidate. I stopped him immediately and said, "Listen, I told you it's your decision, and as with all candidates you can only evaluate what you see. If he tanked, he tanked. If you wouldn't hire him under other circumstances, then don't hire him just b/c I know him; I told you that." He ended up not hiring and later expressed his thanks to me that I made it easy on him to do what he knew was right, but that he was worried I'd be mad about.
Also, here's a nutty idea I thought of in the debate in the other answer's comments: Why not just ask the hiring manager in advance what kind of advice he's comfortable with giving the friend? He or she will likely be willing to set boundaries and respect the OP even more for thinking of it.
This is super long, sorry about that. Hope it's helpful.