What is the proper way to address a widow?

My work consists of me talking and writing to people who have recently lost their husbands. I waver on wether or not to use Mrs. or Ms.

Sometimes I think it would be hard to see the name Mrs. So and So. Other times I feel that Mrs. is like Mr. President. Even when the job is over you get the same greeting.

Usually I use Ms. as the default, but I can envision someone getting upset that they are no longer referred to as Mrs.

So far, I’ve had no complaints either way. Which do you think is correct?