Is it wise to tell parent with Alzheimer's that her husband passed away? She has had Alzheimer's for over 7 years.

This is very tough. My general rule is to tell them once and then go with your gut. You may want to read this story:
https://www.agingcare.com/133806
Telling them about the death of a loved one is what it's about. Just paste it in your browser. Good luck with this. It's hard.
Carol