I definitely wouldn't involve a regular, good customer. It is nothing to to do with her, there is nothing she can do about it and it may come across as gossip. It puts her in an awkward position.
I would leave a note through the door, asking her to pay or contact you by tomorrow... If she has genuinely not got around to it, it will show her it isn't too late to do it now. If she doesn't respond in any way, then obviously don't deal with her again and be careful who you give credit to in future :-) xx