No, if you tell them, then you are abusing them. Or they say you are not a psychiatrist, how would you know?
That is exactly what happened to me, both my pwBPD and the therapists [her individual and our couple's] told me the same thing -- I was a bully and I am not qualified to make a diagnosis -- even though I said she was behaving like a person with BPD.
However, both of the same T's were notably silent when I told both of them the damning traits/symptoms which my wife had already readily admitted to and was aware of. Additionally, I've been validated by two professionals in the 1st person, and two other's in the 3rd person, 3/4 are PhD's, and the one who isn't has 4 licenses in the field, in addition to several other YouTube experts [Dr. Ramani & Dr. Fox, and other licensed professionals who have described my pwBPD's behavior exactly. I am > 99.99% convinced of it -- so that is how my individual T is helping me.
I have since gone covert on my actions regarding the 'borderline' as I suspect the T's are indeed treating for the 'borderline' as her symptoms/traits are improving.
Since I have learned how to work the system upon the suggestion of the T who ghosted us when she recognized the symptoms as valid [she strongly alluded to it] -- send e-mails to the T's, her individual and her couple's T, and list your concerns explaining that it "sounds like" BPD, quote the DSM-5, and give specific examples of each symptom, especially if your pwBPD has said something different in session, and you will have to describe the symptoms by their definitions [e.g. suicide threat/attempt, physical abuse -- both of these are mandated to be reported to authorities, borderline splitting [describe the black/white thinking giving specific examples, and how it upsets you when (s)he does this], disassociation (hard to prove this one, as it is he said/she said -- ask if (s)he can remember from xx:xx to yy:yy time frame -- ask the same question later on in session, at least 20 minutes apart, they may not remember and you can trip them up as they will be making it up and may likely be disassociate), projection of violence [raging, breaking things, hitting things, throwing things, etc.] Whatever you do, tell the truth to the T's, even if you reacted back towards her while she was raging. Both the pwBPD and the T will call you out on it if you don't. Likewise the T will call her out on it as well if she is not forthright.
You need to have a great deal of trust to be able to say anything to them and be functional, let alone tell them they need help.
Look at this way - imagine a person that you think is awful goes to you and says you need help. What would you do?
I agree, I thought I had that 'trust'; however, it was still not enough [however, it was sufficient for my wife to look up the symptoms and read two books on the subject before concluding that she doesn't have it]. I was also asked the question about needing help. All of the T's didn't like my answer of "If she said that of me, I would do a 'deep dive' on myself and likely get a professional opinion of a psychiatrist, which one do you recommend?" One of the T's asked, "if you get a 'negative' answer [while she was nodding towards my wife] would you accept that." I indicated "yes I would accept that, which psychiatrist do you recommend?" Needless to say, none of the T's would recommend a psychiatrist, and my wife after her next individual T's session indicated that she would refuse to go to one [she couldn't deny the suicide attempts or domestic violence and other related behaviors, and it would only hurt her if she followed through with the divorce threats]. To me this is additional circumstantial evidence to what I already know, and they probably know as well, even though they won't admit to it.