under what circumstances is it morally acceptable for a nurse to tell someone that a person that he or she has had contact with or will have contact with is HIV positive?

If you change the word "morally" to "legally obligated," there is no job-related circumstance that I know of that would require disclosure. Everything I know about your profession requires you to keep medical information confidential. Often people ask a question including the word "morally" with the intention of suggesting a higher standard than the law. In practice, the law's standard is extremely high. If you are concerned that a person you are talking with may be putting himself or herself at risk by having unprotected sex or sharing needles with someone who is HIV positive (and how would you know?), then have a discussion about prevention and risk in general. If you do more than that, you are exceeding the boundaries of your professional commitments as I understand them.