Just be honest... tell her that you are at a loss for words but you wanted her to know that you are thinking of her and praying for her every day. And let her know you miss her at work.
I'm really terrible at coming up with eloquent things to write or say when things like this happen so I always just fall back on honesty. Sometimes a simple, honest note just seems right, ya know?
And I also wanted to tell you that I am very sorry to hear about your friend. News like this is always so difficult to understand and accept.
I'm so sorry ...