[NAME, COMPANY AND ADDRESS, ex.
John Smith
1234 First Street
Suite 567
Anycity, Anystate  85245]
 

Dear [NAME, ex. John Smith], 

I have just heard of your [husband / wife / brother / sister etc.]'s death.  It would be impossible for me to put to words the sincere sympathy I wish to express.  I have worked closely with [NAME, ex. Bill] for the past [DURATION, ex. two years] at the office.  [His / Her] passing represents the loss of not only a co-worker, but also a very close friend.

If there is anything I can do for you at this difficult time, please let me know. 

Sincerely,

[YOUR NAME, ex. Jill Jones]