This question is asked out of curiosity. While I ask this using a situation form my own life I'm not asking for legal advice and my actions are not going to be affected by the answers, I'm not actually afraid of being sued, because the individual in question would have to be an idiot to draw legal & media attention to his actions, so feel free to answer without fear of my treating it as legal advice.
There is a certain horrible man were call Joe. This man has had rape allegations made against him by at least 5 women that I know of, potentially more. Some of these women spoke to me in private, sharing their stories so I could warn others about him, but refuse to come forward in public to make the claim, those who work with rape victims will know how hard it can be on women to come forward after a rape, and the circumstances of these rapes likely make it even harder for these women.
I have, and plan to continue to, warned women in danger to avoid him when I see the chance due to these allegations, as I don't want him to harm anyone else. However, I respect the privacy of the women who have shared their stories with me and, while I don't legally count as one, I try to treat them with the same right of privacy anyone speaking to a psychiatrist would receive. I would not share their identities without their permission no matter what.
Thus I'm wondering if Joe were to be foolish enough to try suing for libel or slander when I inform women to avoid him due to multiple rape allegations that I'm aware of (I've warned people through both mediums so either could apply) and I refused to produce the women who shared with me in private about the rapes rather the lack of proof would make me guilty of the crime even though these individuals exist? Realistically I'm sure I could get them to come forward in such a situation, but for this question lets pretend that they refused to and I would not reveal their identities without their choosing to come forward.
Secondly, of the 5 rape allegations I'm aware of I could provide proof that others have heard the allegations, without producing the original women making the allegation, for 3 of them. If I were sued for claiming that i"m aware of 5 allegations and provided proof of the 3 that others have heard of, without revealing any information about the two individuals who confided to me personally and who's identity I wouldn't reveal, could I still be found guilty of slander/libel for making claims I refuse to prove about the two confidential allegations even though circumstantial evidence (in the form of other rape allegations being made) suggest that it's plausible that someone who is known to be an advocate for women in the community that Joe targets would hear of other allegations?