F.H.
I would text back and say, "oh sorry, just confirmed with hubby that we have other plans tonight and can't make it. Hope you have a happy bday". Is this for real? Pretty ballsy if you ask me, if she actually ASKED you to take her out. However, after I recieved her first text "reminding me of her big D.", I would have simply texted back and said, "great, happy bday, I hope its great!" and left it at that. Good luck.