@MariaCecilia22
It looks like you live in the main house on the property which is right next door so you are in close proximity to your listing.
As others have said, I would go over and apologise profusely for finding the cottage in a slightly unprepared state. Just state that it was an honest mistake for which the cleaner is very sorry and offer her a box of chocolates or a bottle of either red or white wine as compensation.
If she said she would like a partial refund you can say...
"You can understand Airbnb get a lot of these refund requests for the most trivial of issues and they don't look particularly favourably on them these days. Most get rejected as its almost seen as a form of extortion, the compensation required doesn't fit the inconvenience, and neither of us want to be involved in that do we. I wouldn't want you to end up with a black mark against your name so, rather than go through the hassle of involving Airbnb I am offering you a reasonable gesture for the inconvenience of running the dishwasher. I hope you can see that, what would you like chocolates or a drop of wine, or is there some other small gesture I can make?"
She will at that point feel a bit of an idiot for bringing the issue up and probably say look it was no big deal and you can then say, fair is fair I want to do something for you! That then makes you look good, puts the guest in your debt.
It does work Maria, I had one guest who on check-in wanted to have a shower and go out and I had forgotten to turn the hot water heater for the bathroom on. He said it inconvenienced him and he felt it was worth a refund. I apologised profusely and told him by the time we had finished talking the water would be hot and would a bottle of red soften the inconvenience. It did and although he gave me 4 stars and mentioned it in the review, no serious damage was done!
Cheers......Rob