I think it's so individual that you can't guess at it or make assumptions based on generation. The fact that she got a divorce yet went bak to live in the same house is already fairly unconventional for someone her age, so all bets are off. To be safe and to not ignore the reality of her divorce and later co-housing (whatever her reasons for it were), I would not use "Mrs. John Smith" at all. I'd either do "Mrs. Mary Smith" (but only if I knew she used that title) or "Ms. Mary Smith" (more likely) or just "Mary Smith" (assuming I already address her as Mary when I'm speaking to her, and not "Ms." or "Mrs."
When she got divorced, people stopped using "Mrs. John Smith" - so the fact that this couple later went back to sharing a house did not reintroduce his first name as part of her official moniker. Being respectful and honoring her are your reason for sending her a card, acknowledging that even though she was divorced, she has still suffered a loss.
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I think it's so individual that you can't guess at it or make assumptions based on generation. The fact that she got a divorce yet went bak to live in the same house is already fairly unconventional for someone her age, so all bets are off. To be safe and to not ignore the reality of her divorce and later co-housing (whatever her reasons for it were), I would not use "Mrs. John Smith" at all. I'd either do "Mrs. Mary Smith" (but only if I knew she used that title) or "Ms. Mary Smith" (more likely) or just "Mary Smith" (assuming I already address her as Mary when I'm speaking to her, and not "Ms." or "Mrs."
When she got divorced, people stopped using "Mrs. John Smith" - so the fact that this couple later went back to sharing a house did not reintroduce his first name as part of her official moniker. Being respectful and honoring her are your reason for sending her a card, acknowledging that even though she was divorced, she has still suffered a loss.