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

Dear [NAME, ex. John Smith], 

I was quite saddened to hear of [NAME, ex. Bill]'s death.  I hope that you will accept my most sincere sympathy and condolence.  [NAME, ex. Bill] was such a well-liked person that it is difficult to accept that [he / she] is not with us any longer.

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

Sincerely,

[YOUR NAME, ex. Jill Jones]