Q&A: Invitations: Addressing One to a Widow or Divorcee?
How should you address an invitation to a widow? What about a divorced woman who has retained her married name? Last question: What about those who are bringing significant others that do not live with them? Can I send just one invitation, or do I have to send one to each of them?
A widow is traditionally addressed as Mrs. John Jones, but if you feel the guest may not want to be addressed that way, it's completely okay to ask her how she prefers to be addressed. A divorced woman who has kept her married name should be addressed as you suggested -- Ms. Jane Johnson. For a couple not living together, technically you should send each their own invitation, but it's not horrible to send an invite to one of them (say, the person you're closer to) with both names listed alphabetically (each on its own line) on the outer envelope.