When addressing a sympathy card to a widow do you use Mrs. her name or Mrs. his name?

If you are writing to a widow whose last name is different from her husband's, you should use the title "Ms." with her name ("Ms. Brown," not "Mrs. Brown"). Even though she may use a different name from her husband's, it is unlikely that she would be upset if you used her husband's name ("Mrs. Smith" instead of "Ms. Brown"). After all, that name connects her to the man whose loss she is grieving, so she might like it.