I’ve worked in LD for quite a while.
Mental Capacity is not fixed. It can fluctuate with age, environment even time of day. It is always situation specific.
Legally, the assumption is that someone HAS capacity unless you have good reason to doubt that. In which case you just assess their mental capacity and this must be recorded. This can be a lengthy process as every effort has to me made (and you have to be able to prove) that amity tried everything reasonable to enable the person to understand the situation and communicate their decision (even if their decision is not what you would choose yourself).
This lady may well be perfectly able to make decisions about what she wants for dinner, where she wants to go every day, etc but not have capacity when it comes to complex medical care.
So at some point, when the issue of telling her or not has come up, she has been assessed and it’s been decided she lacks capacity in this area of her life.
It then becomes a best interest decision, which is also a formal legal process. The mental capacity assessment done prior has to accompany this best interest documentation as you can ONLY make best interest decisions where someone has been proved to lack capacity.
Again, the best interest decision with be decision specific. In this case, it will have been ‘should X be told about her terminal diagnosis’.
People involved in her care and her life would have been involved. This includes next of kin, her doctors, sounds like your care manager (who arguably should have asked for your views also if you work closely with her and know her best), anyone with legal guardianship (so her SW). They will have looked at the benefits and downsides of telling her or not, and weighed the decision based on what they felt was in her best interest.
She does have a right to know yes. But that does not necessarily mean she NEEDS to if the consequences of that for her would significantly put weight any benefits.
For many of the people I used to work with, knowing this information would not have been beneficial for them. This is not a neurosurgical person with no LD, hearing devastating news, being able to process it, come to terms with omit and then getting on with making the best of what time they have left. Many had poor concept of time for example. They would have been asking every day when they were going to die, is it today, is it tomorrow? They would have told everyone they met they were going to die. It would have occupied their every waking thought and caused fear and anxiety that just would be unnecessary and cruel because they were not able to process the information in any helpful way. It would have tormented them until their last moments and they would have lived their last months in fear and confusion.
In your situation, I would suggest speaking to your manager and asking to read the best interest decision and capacity assessment so you can get a better understanding of why this decision was made.
If you feel that the capacity assessment was un-sound (for example the lady uses PECS to communicate but PECS weren’t used in the conversation) then you can challenge it and it would have to be re-done. But if it’s all been done properly, with due care, then you will have to accept the decision. But hopefully being able to see how they weighed it up and reached their final decision will help with that.
Do reach out for support for yourself as you continue to support this lady towards the end of her life. It’s wonderful to see your care for this lady, it comes through clearly in your posts. But don’t let your grief and emotions about her prognosis cloud this. It’s about what’s BEST for her, emotionally and physically, what benefits her most.