​Client won’t pay for tile job

I recently tiled a bathroom and did some other work for a client that I’ve worked for many times before. She has not paid me the $1600 she owes because she claims I tiled over the floor vent in the bathroom.

Even though there was no vent or piping, only rotten floor joists And debris that I repaired/cleaned up.

Since we have done business multiple times before; this was just a verbal deal, but now I feel I have no recourse as I can’t prove we had a deal and she won’t pay.

And she is hell-bent on the idea that there was a floor vent there despite no piping under the bathroom in the basement; other than water drainage/supply, but no HVAC.

I’m at the end of my rope here