Sympathy for Co-Worker’s Death –
[Name, Company Name & Address here]
I have just come to know that your [relation with the dead person] is no more among us.
It was saddening news and, I send my sincere condolences to you and your family. He was a great person. I worked with him/her for the last [duration] in my company. He was very helpful and supportive for everyone and had many qualities. He was just not a colleague; he was also a good friend of mine.
Please let me know if there is anything I can help you with at this tough time.