She is a recurring character since the first season. She knows Betty is married, in fact she played a part in connecting Betty to her new husband. I think it was just a dig at Betty calling herself "old fashioned." Francine called her "Betty Draper" right after Betty did a "woe is me" for being so old fashioned. Right, finding a new sugar daddy before leaving your liar of a husband is so old fashioned. The whole lunch was typical of their strained relationship. ""Oh, your time is occupied" was a play on her mixed emotions of Betty's youngest child and her own empty nest issues.

I don't know if it was an intentional faux pas on the show--because of course she would have taken her new husband's name in that era-- but it doesn't strike me as strange that a close friend might still mistakenly call her by her former surname.

She is a petty, mean spirited woman and her friend was right to insult her teasingly, especially after Betty spent the entire scene slamming her for her working.