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.