for context the friend's sense of betrayal might relate to the years-long sexual relationship they had with this person, and a sense of injustice because the sexual partner is aware of how the HIV is being treated, that the viral levels are not perceptible in blood tests, and that the HIV's risks have been mitigated such that it shouldn't pose a health risk ... I think the friend also views the polycule's rule as ignorant about HIV and its treatment.

Could just be a personal thing on their end, but I wanted to get a pulse check on how common a rule like this is, as I am familiar with the queer community's attitudes about this (basically HIV is no big deal and is mitigated, people who are terrified by HIV are usually a straight cringe stereotype, etc.), but I don't know whether that attitude overlaps with the polyamory community or not (I assume the overlap of queer and polyamorous people is significant, but maybe I'm wrong).