Bloody hell
I'm so sorry for you and her.
First of all you are lovely to write to her. So many people duck out of this tricky task and from what I've read on MN, letters and sympathy cards really do make a huge difference to those receiving them.
quick google found this which is probably along the lines of what I would have written I think.
Other guidelines from t'internet include: -
Start off to say how sorry (or shocked, depending on the nature of the death) you are to hear of the death of the person and mention then by name.
Say that you are thinking of them and the family at this difficult time.
Depending on how well you knew them, it is good to note any special qualities that the person may have had.
It is nice to include a good or happy memory you have of the person that has died. It may be that the person you are writing to isn?t aware of your memory and can add it to their own treasured memories and have a smile about it.
You might want to mention the qualities of the person you are writing to that may help them through this time. You might also want to mention the happy memories of the deceased that will help sustain them in the future.
Offer help if practical, but make sure it is specific and not the ?ring me if I can help? type of offer. This may be just to drop a meal around, do some shopping or help with arrangements for the funeral.
HTH and good luck with writing it.