I was the around-the-clock caregiver for my parents, for the last several years of their lives. Both passed about a month apart, last summer. Mom, had Dementia/Alzheimers. Dad, stage 4 cancer. As horrible as the cancer was, at least he knew who he was, right up until going into Hospice in the hospital, two days before he passed. He came around for a brief moment right before passing.
Mom's condition was just cruel. Scared, anxious, not knowing what was happening from moment to moment. Fortunately she knew who dad was, and who I was most of the time.
I found out that being a caregiver is something lots of people will not do. My two older sisters who both live less than 20 miles away, couldn't be bothered. One is a Social Worker...
Your wife will never regret the time spent in her Mom's time of need.
My sincerest condolences to you and your wife.