What and/or how would you suggest I tell someone if I suspected they have BPD?

There is a person in my life whom I care for deeply. I have only known her a few months, but we (my DH and I) are in a polyamorous relationship with her husband. The more I get to know her, the more and more I wonder if she has BPD. She hits literally every single symptom in the DSM. I also recognize how similar the traits are to some of my other loved ones who were actually formally diagnosed with BPD.

Now, I am fully aware I am not a psychologist, and have no right to try and diagnose someone else's mental health problems. However, her husband struggles with her emotional dysregulation. He gets overwhelmed and unsure of what to do sometimes. Sometimes, his reactions make things way worse. And now, we have also been on the receiving end of her struggles. Sure, any regular person would say " WTF are you doing? RUN". But because I've seen these traits since she started allowing her true self to be seen, I struggle with dropping her like everyone else in her life has. I want to support her the best way I can.

She sees a therapist and has already put in massive work into herself over the last couple years from what I understand. And I know that there is also massive stigma about having a BPD diagnosis. I asked her husband at one point if he knew what her diagnosis was, and he wasn't really sure. He just knows meds help. So, it's possible her therapist already knows and has elected to not tell either one of them... (This is what happened with my family member that has it. Everyone was told, except them.)

I feel like at least telling her husband what I suspect could arm him with better info to help and support her and help her through her meltdowns. And it would also make us better equipped to help her if/when we need to be involved.

What's the general consensus these days?

• Keep my mouth shut and do my own research pretend like I don't have my suspicions, but model my behavior around it?
• Tell just her husband so he can do his own research and stress the impact that telling her may have?
• Sit them both down when I know she is in a higher mood, and let them know?

If the advice is to tell them both, how do I approach this in a way that is well received? I don't want to just blurt it out as unsolicited advice, because that's just rude and no one takes well to that.