I sent a card. I told them that I was so sorry for their loss. There wasn't anything else to say that would be genuine so I found a poem by Edgar Allen Poe that seemed to fit well.
Thank you all so much for sharing your experiences and offering advice. It really has helped me a lot and I will probably come back to this thread again in the future. There is so much good information about what to do and what not to do in a really difficult human situation.