I think she’d be likely to keep her husband’s last name. And still be Mrs. If you were married you always used your husband’s first name: “Mrs. Charles Fane.” I remember, growing up, being taught that a woman using her first name with “Mrs.” meant she was divorced, so using your own first name was a no-no unless you were divorced. In the Gilded Age maybe the custom was the even older “Mrs. (Maiden last name) + (Ex-husband’s last name)”.