Should I warn new/prospective PhD Student that supervisor is terrible? My supervisor got some funding and is hiring new PhD students. Today one of them came to an interview and we told him about the things that we do.

The main problem is that my supervisor is a terrible supervisor. He wants to lead the research project but has no idea on how to work on real implementations. He does not help or even provide feedback although he still is the leading author of all the publications we do. He does not want to spend money on us or on equipment, but expects results as if we were using high-end equipment. He treats us plainly wrong, taking credit for our accomplishments, humiliating us when possible, not helping or leading in any way and not allowing as to grow as researchers.

I know that I could be biased for some personal bad experience but 100% of the students at my research center (both his students and other students) agree with me that he is, by far, the worst possible advisor that there is in our research center.

My question is: should I warn a new coming PhD student about this so that he can take his decision taking this into account, or am I supposed to say nothing because it is not my business?

On one hand, when I was where he is now I would have LOVED if someone told me about this situation, but on the other hand as a PhD student I'm risking a lot by saying this and I don't know if I should get involved.

EDIT: At the end I did it. I called him as it was impossible for me to see him face to face. I basically explained him several facts (not opinions) and my personal view of the situation, trying to be as much understanding and empathic as possible. Luckily he has done research before and understand me very well. He thanked me for the honesty. On one hand I feel that I did the correct thing, but on the other one I think that this situation is a time bomb that could explode at any time... But today I slept like a baby, which I think is a good sign. Thank you all for your answers, they helped me a lot.
You should. As you mentioned yourself, you would have loved it if someone had warned you about things before you went, and "do unto others as you would they do unto you" is a good maxim to live by.

Having said that, you should also take steps to protect yourself. Talk to the student in private, and ask for confidentiality. Do it face-to-face or via telephone/Skype if you can; if you must use email, avoid using your institutional email address. You've already acknowledged you might be biased, so tell the student the facts only and don't add anything that you inferred from the facts. For example, instead of "He does not help or even provide feedback", say how many times you meet him or attempt to contact him a week. Describe how those meetings went - what did you say, what did he say, what did you do next? Try not to say "he does not help" as though it's a bad thing: it's actually possible he is expecting you to work independently for various reasons (e.g. you were so good he doesn't think you need close supervision).