A friend of your X. My suggestion is to drop it and move on. At least you have the upper hand if she asks you to clean again - its her fault for not paying you.
OBTW, she can easily call up her last few months bank statements to see if the cheque has been presented for payment.
I had a cheque delivered to the wrong address last month and the home owner kindly hand delivered to me, so there is a possibility that she did post you a cheque and it didn't arrive, albeit a very slim possibility.