Cognitive issues with MS...common but there is hope.
It can come on gradually and she may not have noticed or maybe she is aware and she notices but hopes that no one else does.
Where does hope come in? There are cognitive tests that can actually help retrain your brain. Her neurologist should be able to direct her to the right person to do the main test to find out where her weakness would be...whether it recognizing faces, numbers, pictures, etc and take it from there.
How to approach her? Be honest and start out by asking her to explain her personal experience with MS as you want to learn more BUT before doing so...brush up on latest MS info...from bladder / bowel issues all the way to cognitive issues and what the latest treatments would be.
You know maybe she is scared that if she admits to her children that she has a cogntive issue that her freedom might be taken away.
I had a cognitive test several years back and it was interesting and I was able to see my strengths and weaknesses and I don't let my mind stay idle...
Other things can cause cognitive issues...has she had her B12 checked or any other labs? Some medication can cause memory issues.