[Your letterhead, if desired; if not, your return address]
[Date of letter-month, day, and year]
[Recipient's first and last names]
[Street or P.O. box address]
[City, State ZIP code]
Dear [recipient's name]:
Thank you so very much for helping me with household tasks, errands, and cooking during my family members' recent illness. I'm not sure how I would have managed to get through this trying time without your able assistance. If you ever need help in a similar situation, I would be happy to return the favor.
Thank you again for your loyal friendship and help.
[Sender's first and last names]