Should I disclose persons HIV status?

Hi, all. I have a situation where a friend of mine disclosed that her younger sister(20years old) is HIV positive.She disclosed this to me and another friend a few years back.Thing is, this girl does not disclose her status to the men that she sleeps with unprotected.She has already had two children with a man who unfortunatley never knew. She is pretty promiscuous and is now with a guy that we know,not personally, but through ociates.Sholud me and my other friend somehow let this guy know what's happening? Is it against the law for us to do so? All advice would be appreciated.