My mother in law's memory has gotten worse in the past few months to the point that she sometimes does not remember her husband. This scares her. In these instances she does still remember my husband and our phone number. We are grateful when she does this as it gives us a chance to reassure her and even get in the car and come over. Fortunately my father in law is still very sharp but it is painful for him to go through this. He adores her and would do anything for her but feels helpless during these times.
Maybe your father-in-law would benefit from a community like this one online? Does he use a computer?
Does your FIL keep photos at the ready to remind his wife of who he is? Showing her pictures of her family may help calm her. I would suggest pictures that span over years. Some from when they were younger, some when they had a young family, some when the kids were grown, and some with grandchildren. This may help calm her and help her remember at the same time. Until the next time.
I can't think of a thing that would help. God bless you all.