Worried by AD with neurodivergent issues in destructive relationships

Does she want/take your advice? Is she merely venting?
She is an adult and my first instinct is to say let her get on with it.
However if she is using you and her friends as therapy then perhaps look on this as a coaching conversation. Google growth mindset.
Ask open ended questions which require an answer from her. Do not criticise her decisions or give her advice, but help her to make her own decisions.
For example, when she phones you crying about the way she is treated think about using the following:
What did you do after the last time he spoke to you unkindly?
How successful was that approach?
If it wasn't successful, what do you need to do next?
How will you go about that?
Which professionals could support you?
How would you access their support?
Then summarise her answers to you. Ask her if that sounds accurate. Then praise her for making decisions and telling her you look forward to your next chat.
It does sound as if she hasn't really grown up and isn't used to making her own decisions. This might need oodles of patience and repeated frequently, but being emotionally detached from her situation can only help.

Make it clear from the outset that you are not going to tell her what to do

That makes a lot of sense, Grandmabatty.

She is in a job where she should have access to help?

Tell her you have tried to help and advise in the past to which she has taken no notice so you are now staying out of her relationship and personal affairs.

You could ask her what she would do if you asked for her advice in a similar situation - i.e. you are being mistreated and upset by someone etc........ask how she would deal with it.

It is irelevant which labels you want to give her, ADHD, ADD, autism and narcissistic are the ones you list. She is who she is the label will not change that.

Tell her to end the relationship or stop complaining about it. Some people seem to just like the role of victim.

She will learn eventually... I would tell her that you know she is going again but that you will keep your own counsel from now on..

Is he being unpleasant because he's tried to end it and she won't? Then she wants to end it but doesn't?

Maybe they both like drama because I can't see what else either of them are getting out of this. The problem is you can't solve it for her. Even if she is diagnosed with something unless she lacks capacity it is up to her, well up to both of them.

It is hard to watch are children being unhappy even if they are all grown up but we are helpless.

I used to find it hard to let go of abusive men but realised eventually that it was due to my fear of abandonment. I wouldn’t tolerate nastiness from her and it doesn’t matter what label you choose to give her.
I’ve got some of the traits you describe but never considered I might have ADHD (I haven’t.) IMO it’s being hugely over-diagnosed these days, especially in adults.
If she doesn’t listen to your advice you need to step back and stop giving it. Don’t cut her off but be clear that you can’t help her unless she learns to help herself. One would hope she’d have some self-awareness if she’s working in mental health.

I would tell her to sit down and make up her own mind as she is one of the only two people involved in the relationship.

As a mother, I would never, never never, ever, get involved in an adult child's relationship. Given from how you describe your daughter. If you have any say, any opinion, behave any way than completely neutrally, you will be blamed. You will be blamed if you stay neutral as well, so gpo out and get you comfort blame slippers and throw and make yourself comfortable. Wrap your self up in them and stay stum.

OMG, thank you ALL for your messages.
Grandmabatty, I love your clear way to talk to her and I'll follow your advice. She hasn't grown up really, and I don't think she ever will, not in the sense that we generally mean. She has had lots of opportunities to make her own decisions in life, but somehow her need to ask for affirmation when making big decisions has increased recently.
She does have 'supervision' in her job but that is mainly to do with talking about dealing with clients and their needs. But there is some emotional support for her needs as well and she does access it. But maybe doesn't carry it over into her personal life...You'd think that working in mental health she would have more self-awareness, but somehow she doesn't or doesn't act on it. Quite frustrating.
LaCrepuscule, I smiled when I read 'if she doesn't listen to your advice step back and stop giving it' as I have learnt over the years never to give her advice! She frequequently used to say 'DON'T give me advice, I just want you to listen!' So now I just listen, and only give suggestions if asked.
I'm so interested to read all of your comments, and am thinking hard about them. Everyone's input is so appreciated.

I feel for you Indiana. Listening to someone’s continuous “woe is me” is very draining. Some people (with a label or without one) are just high maintenance.

Does not seem there is much one can do here , except being an open line of communication. Only the worrying part which country this man comes from and does she sends him money and expecting the relationship to turn into what because right now being long distance....kind of obscuring the end goal of it