Polyamory

People need what they need to feel safe, and as long as noone was coerced, but agreed willingly to this particular set of STI risk management rules, I don't see an issue with them. And it seems like your friend's friend has done so. If they didn't want to give up sex with your friend, then they shouldn't have entered a polycule that has rules like that.

I can definitely understand that your friend is hurt, though - they've basically been told "My new relationship is more important to me than our existing relationship", and I think it's pretty weak sauce from your friend's friend that they try to put that on the polycule's rules (like "I don't want to, they're making me do it, I have no choice!" bullshit) instead of owning it as a conscious decision they made. I personally would feel most betrayed by that kind of dishonesty.

Edit: I thought this was a post on a different C/ when I responded. I'm not poly, so I wouldn't normally respond to something here. My apologies for not paying better attention, but I'll leave the comment here in case it's useful despite that.

It's not limited to any given relationship structure. People either are or are not okay with a given health issue that may impact all parties involved.

When you're dealing with a structure as complex as even a three party relationship, the degree of discussion and communication necessary to maintain stability is high. It isn't at all unusual for the core group to have agreements about hard noes, and a very common one is STDs. With HIV currently being an infection that requires long term treatment, and often would require going on PREP for every member of the group, it makes total sense. A lot of polycules require regular testing and/or testing before joining.

I'd be surprised if that was the only STD/BBD that they have a rule about.

As far as an opinion, it makes sense. Everyone has to decide what limits they have regarding risk. Maybe you're okay with herpes, but not syphilis. Or vice versa. You gotta decide for yourself. But, when you have multiple partners, their decisions can affect you before you have the ability to make informed consent if you don't hammer it out together. So a polycule that's open to members having sex outside the group kinda has to work with the most restrictive limits members have. Any new members would then have to adhere to those limits or not join.

I'm not poly myself, though I'm not against it and wouldn't automatically discount a relationship with someone that was in a polycule (or wouldn't have, I'm monogamous in general and am married, so no outside stuff at all). But I would damn well need to know that there was some kind of understanding about risk mitigation in place.

Which, I have a fairly long history of dealing with HIV/AIDS issues. I was around back before AZT was even created, and it was new when I started work as a nurse's assistant. There was a time that I was the only NA willing to work with AIDS patients at all, and one of a small number that would work with HIV positive patients that weren't symptomatic. Back doing my clinicals, it was optional to train on the HIV ward, students could just opt out. I was one of two in the class that didn't.

Even with that, it would be a big ask to engage in sex with someone that was positive on a regular basis. And an even bigger one to invite someone into my marriage that was positive. I'm not saying hell no, never. Not with the modern meds that can keep a person undetectable, and PREP being an option. But if that went away? Hell no. I've seen that path play out, and I wouldn't take it. So I can't blame a group not being willing to deal with the risks, no matter how small

By definition, polycules include people who all members are cool with, and exclude everyone else. If a member has cancer or is otherwise already immunocomprimised, excluding people with STI's makes sense.