Can you ask your son what is your DIL's favorite meal? My next move would be to invite them over for dinner and cook them a feast. But when they come over, try not to mention anything about "A" being separated from you. I wouldn't bring this up at all.

Then, the following week have them come over for your son's favorite. While DIL is over, try to get to know her and find out what her life was like growing up. The only way you'll understand a person is by getting to know their life as a child.