There is a very high chance that this is simply what my friends and I call a Facebook!Fail.
I have dozens of people blocked, limited profiled, and put under specific limitations. If I refuse to let them comment, they cannot see to comment. If you can't write on my wall, you don't see my wall. If you can't comment my statuses, you can't see them. That's how the FB privacy settings work.
However, I get that notice a lot too. If you're really determined to do this over FB, try writing a message. It's more private anyway, and if you guys mend your differences long enough to express grief, she not want all of her friends to see just how much she's hurting.
I agree with PP's that say this goes beyond FB and deserves a phone call at least, but if you think she wouldn't pick up or would ignore a letter from you unless you left the return address off the letter, then the *only* good thing about doing it over a FB message is that you can include a subject that is seen as soon as she goes into her inbox and the first part of the message itself.
If this is the only way you think you can express condolence with her recieving the message, that makes the subject and first 30 or so characters extremely important.
But my vote is for a more personal method.