Hello Cartman, and welcome to TP.
Before my wife came down with dementia, I never liked to speak about personal issues, except to her.
These days, I talk to all and sundry about her illness - if they show interest. It helps me, it helps me to help my wife, it helps the other person to understand why we have changed, and it helps me to know that the friend understands why we have changed.
The transition from not talking about it to talking about it takes time, and it also depends on your relationship with your close friend.... how far you feel you can intrude without being invited.
For my part, I was relieved when the odd true friend showed they knew and cared.
Dementia at an early age is dreadful - my wife was about the same age as your friend's mother when she started to show symptoms.
Sometimes friends need to take risks. In your place I would pretty much do what you say - "let her know I know and that I'm here to support her and help her in any way and in her own time, when she is ready ". Tell her that, and leave it at that, unless she asks you in, as it were.
You have already elevated yourself to the status of true friend, by even thinking of this. Congratulations, it ain't easy!
Best wishes