To follow up on this thread, I would like to add that my wife says that the information gleaned has been far more than just useful and has in fact had a very positive effect on how she is dealing with the situation. She has been reading up on the links provided and is becoming increasingly convinced that the friend is indeed not on the Asperger's spectrum but definitely shows all the traits of DPD. The valuable part of this is that she can learn the appropriate responses and not get stressed by not knowing how to behave. As an Asperger's herself, this is immensely important for her, because social interaction is usually a big problem.
I'll spare you most of the detail, but the picture emerging is made up of behaviour which now fits a clear DPD pattern. For example, this week the friend has to undergo relatively trivial surgery, and a third person kindly offered to drive her to hospital. But she didn't want that - she wanted my wife to drive her, be with her until the operation and be there when she comes round from the anaesthetic. Now that she knows what is going on, the wife actually took a step back, and is prepared to take her home when released a day or two later, but that's it. The friend also requires continuous wall-to-wall sympathy via social media because of the impending surgery and associated dietary restrictions, behaving as if she were having open heart surgery instead of a gall bladder removal. ("The odds are in my favour..."). The wife has not responded to the pleas for sympathy in an overtly dramatic manner, so friend is sulking. The distancing starts here, but it is difficult to do without seeming unkind.
It does seem unlikely that the woman would agree to more therapy to help with a self-awareness and enable her to have more normal human contact. As much as I would like to help this woman, I don't see what we could do if she is not willing to help herself, and my main concern is ultimately the effect she has been having on my wife.
Predictably, the wife has suddenly become angry and resentful after a few years of frustration, so the past few days have been a bit of a white-knuckle ride, and I have been going on long solitary walks. But that is only temporary, already improving, and a necessary part of the process, and she is delighted with the revelation and the situation can only get better. So thanks again.