Talk about the person who has been lost. People tend to not want to "bring him up" for fear that it will trigger emotion in those left behind. Nothing could be further from the truth!
Let her & her family know that you are thinking of him too. Share memories, etc. A good friend of mine from HS lost both of her kids last summer in a car accident. She has said so often how much she appreciates my continued support and reaching out. I had never met her kids (we live a few towns away from each other), but being a mom, I can imagine the pain. Reading things on her kids FB pages has given me insight to their lives. So even if you didn't know the person, talk about him. Ask questions about him. Let them know you care.