Nothing. Nothing from the day I met her. She never introduced herself, just made it known she knew who I was (she came into my work). When we broached the subject of what I should call her year's later, she told me I was to call her "DH's Mother" (obviously DH's real name). As both DH and I found that utterly ridiculous, I continued to call her nothing. Now there is no issue anymore, because we don't see her.