What a fantastic question.
One which unfortunately we will all have to deal with at sometime, If we haven't allready.
I personally would'nt visit, or call round, unless it was a family member.
Grief is a very personal matter, especialy if the person involved is just known to you, and not close to you,
A beautiful card with warm words such as " I can not imagine how you are feeling right now ",
Or, My thoughts are with you,
Or, I feel helpless at this time, but I'm enclosing my mob no, please texed your name, and I will know what to do...
I could go on for hours, what a great Hub.