Should I refer to a widow as Mrs., Miss, or Ms.?
Although there are no legal, grammatical, or lexicographical rules governing what courtesy title is "correct" for a widow, in general, when a woman's husband dies, she retains the title of Mrs. So-and-so.
Widows, like anyone, ultimately have the final say about what they wish to be called, and there are certainly widows out there who prefer the title Ms. (or even Miss). But, when in doubt, stick with Mrs. — or just ask.