Passing of a co-worker

I found out today about someone who, at the hospital where I work, had died over the week-end. It was sudden to say the least. No one knew she was  sick. She had confided once to me that she had Crones. A disease affecting her intestines. Not more that two weeks ago she went in for tests thinking it was related to her disease to end up having a biopsy of her liver, to full blown liver failure and her passing over the week-end.
What to say,
It is customary where I work that the management places a photo of the employee , flowers and a book for everyone to write their condolences.
I always have a hard time with the writing of condolences, words are so shallow, they can never express enough, or take away the loss.
I had met her son once, I feel compelled to write something, so I imagine I will. Although what ,
at times does escape me.