Honestly, either your girlfriend does all the cleaning. Or charge a cleaning fee. You were informed that the dog is potty trained. Maybe being in a new place is scary, or whatever. But it’s not your responsibility to train her dog to not spoil your home. She is responsible of the clean up cost.

Give the dog back. You didn't sign up for house training the dog.

some cleaning solutions have amonia in them, which can make dogs pee indoors. my old dog wouldn't pee on the floor, even if i didn't take her out for a long time, but she couldn't hold her pee for more than 3 hours at my work because of the cleaning solutions they used.

Charging a cleaning fee would be good. Also, for future stuff like this, it's always a good idea to have a visit or two with the dog in the new environment prior to the petsitting. Sometimes the stress of the change throws their training out the window