Bear in mind that the decision not to tell her isn't a 'once and forever' decision. If her situation changes, the decision may change. It may become clearer in future that she does want and need to know.
I looked after children with emotional difficulties who needed support to process difficult things. There were times when we didn't tell them about coming events until the last minute, in order to allow them to enjoy each day. If they knew about a holiday booked, for example, they'd become very erratic- unable to sleep, over excited, anxious- and day to day life would be trying for them and us.
We'd trail what was going to happen without actually announcing it. Lots of work was done on preparation, then at an appropriate time beforehand we'd give the detail. So we'd buy clothes 'for the summer', talk about singing songs in the car 'if we were on a long journey' etc. When they found out about the trip, they already knew they had appropriate clothes, games for the car, etc.
If they knew too soon, it would stop them coping with day to day life and they'd have no benefit of the knowledge. Perhaps it's the same for your lady. Perhaps the people who have made the decision have seen similar situations before that inform their action.
I'm sorry for your sadness. I'm sure as she becomes less well, there will be opportunities to support her helpfully. It's very sad and she's lucky to have someone who cares about her as you do.