How dies she pay (when she pays)?

If cash I would respond. Your receipt is sitting here, when you bring payment you can collect it. I would also consider something along the lines of "when you disappeared without payment I no longer knew how to get your receipt to you so was waiting for you to be in touch about your owed fees so I could get it to you".

I would back date the receipt to when she dissapeared then i would hand write a note on the receipt. A note to you/CRA that you lost contact with her and did not know how to get the receipt to her. 'Unable to locate-lost all communication abruptly".

I doubt it would hold in court but it makes it appear it was their dissapearing that resulting in a late receipt. Not you purposely with holding it.

That said, you DO need you get it to them whether paid or not. BUT I don't know there is much they can do if you kind of attempt to hold it until paid.

If you do NOT have a signed contract you are likely best to just seek the money for days care was provided. Some money is better than no money.