So.
Kind of a long read.. 

Not really possible to tl;dr.
Sorry.

I have a good friend with BPD who I really love and care for.
I've been spending time with them and was in a relationship with them for 6 years until she had a breakdown after her dearly loved uncle and cat passed away. Her mental health visibly started to decline and she started hanging out with a Person who she's friends with since childhood and it's clear that Person manipulates her and brings out the worst in her. She embraces and encourages her maladaptive behaviour and supported her in breaking up with me to go try out an open relationship with her and started hanging out with weird dudes and just completely lost it in so many ways. Clearly she wasn't herself anymore and she needed help. She almost broke away from anyone usually near and dear to her.

Weirdly enough we still had contact Afterwards because she was afraid of losing me. She later said the break up was actually a Trauma to her. 

She started moving in with that problematic Person and one day all of a sudden mentioned to be sure she has DID (multiple personalities).

Gotta mention: she's had a professionally confirmed Borderline diagnosis for about 4 years now... 

She was close to starting her DBT sessions and almost threw her place on that waitinglist away because she was convinced her real problem would be DID.
To elaborate and explain that she does think she has it she mentioned the following reasons.

She could kind of hear voices around her. 
She doesn't feel like she knows who she is.
She has memory problems.
She dissociates a lot.
She has already "seen" and "heard" her different personalities.
She feels like a different Person depending on the Situation. 
She says it's loud inside her head. 

As an outsider (who sees the way she changes in her way of talking acting and thinking) she is an absolute prime example of Borderline.
She hasn't shown much of it in the 6 years we were together (I assume it was quiet bpd that she had to fight because it was hard to recognise those symptoms). Mainly because my family and me gave her a place to live. A healthy Lifestyle. Everyone loved her.
Basically something everyone could only wish to get...
She Was getting better.
Until that Person came back into her life and started talking her into self destruktive behaviour.
She started taking Drugs. Weed at first and it was already more than noticable what effect that had on her. She talked about psychotic phases (not sure if Weed induced). She started talking about wanting to take ecstasy and her girlfriend said it'd be fine and not that extreme if you took anti-depressants. (extremely fucked up Statement if you ask me.)

The reason I think she's making up did and needs desperate and different help is mainly the Person manipulating her (besides me having witnessed her behaviour over the daily course of almost 6 years....) 

She often seems to invent illnesses and kind of seems to have every illness on this Planet. (some of them supposedly confirmed although from a Timing and Personality perspective seems unrealistic since she often left the clinics she was in, didn't feel the need to see therapist, just didnt want to see them or moved towns so there actually was no time for the amount of Diagnose she has.) Oftentimes she says she feels the same symptoms as my ex PwBPD and that she has the same illness. Both of them talked themselves into being autistic at some point for example (which was pretty clearly bs tbh) . Both of them supposedly have BPD. Only my ex PWBPD has it confirmed though (afaik).
And whenever both of them start to self Diagnose some illness they start to absolutely lose themselves in the topic.
They are 100% convinced of their bs and absolutely really celebrate their non-existant illness.
Both of em are very creative, love drawing and love to do fictional RPs with their own universes and characters. (which I think led my ex PWBPD to lose a bit of her sense of reality). Regarding the autism story they started to invent funny toony characters around that for example. 

When she showed me the list of "persons living inside her" I kinda didn't know wtf to say or to think.
A lot of the 20+ personalities (which she all found within the span of around 3 days) are, as I had recognise, many of her OCs from the RPs with her gf.
Same names, relatively the same description of character and interest...

At this point it was just obvious to me the she must have actually lost it now and I was afraid of the enourmos toxic influence this other girl had on her...

I don't know where to go from here.
I'm absolutely sure she does not suffer from DID and her family and loved ones also don't see any behaviour that would show this even slightly...

All that she's holding onto is the fact that I told her about 3 years ago that her mood swings make her seem like a different person (when I didnt know wtf BPD was) and her therapists having mentioned a single time that she would also suspect her having DID.
Also that her style of therapy would suggest she trained her to handle DID instead of BPD.

She already started therapy for DID with a new therapist now and doesn't seem to want to attend DBT it seems... She wants to be refered to as a System and keeps saying "us" like there's multiple people talking. 
She's been reading the subreddit to DID so she could easily adapt the behaviour and thought processes so a therapist might even believe her and Diagnose her with it... She absolutely convinces herself of this non existant problem. 

I think.... 

talking herself into having DID is just what she needed.. At that time.
She Was completely broken.
Couldn't handle the emptiness. Couldn't handle not knowing who she was... 

It was easier for her to say she's everything at once instead of being none of it... 


Like..
Where do I go from here?
Am I an asshole for claiming she doesn't have it, no matter if I'm 100% sure?

Do I tell her?

How do I tell her without making her angry and have her retreat more from healthy persons around her?

All of this is so difficult.
I just want her to get back on track.
I want her to get better....
She was so close to living a complete and healthy life and just threw everything away in the span of less than 2 months...

Anyone got an opinion?

Thanks a lot for reading if you made jt to here..