I agree that HR emailed her yesterday asking her to come in to see them as soon as she arrived and they told her to sort it out. The cheerfulness was a mask, as was her response to you. If she sorted her internet banking yesterday, then she could have sent you the money then, and if she intended to give it to you today, she would have brought in the cash and given it straight to you as soon as she came in. She had no intention of paying you today, until HR pulled her up on it.
Have you decided what you will do about Friday Lunch Club going forward?