Sympathy Card Cancer How To Write A Sympathy Card?

How to write a sympathy card? - sympathy card cancer

I work for a professor, and his wife had cancer and died a few days ago. The ceremony took place today, but I could not go because I had an exam. I want a sympathy card, but what should I write? I did not know his wife before I do not want something that seems very strange to write about.
Thanks