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.