What is the proper etiquette in addressing a widow?

A widow is Mrs. Married Name unless she specifies a desire to use her maiden name or legally returns to her maiden name.

It is not improper nor insensitive to recall through the use of her name that she was married, in fact it may be considered insensitive by some widows to use Ms. or revert to a maiden name, which is never done unless specified by the widow herself, as they may see this as an attempt to ignore years of matrimony or the passing of their husband.