@Helen386
well, if he cancelled and now request full refund from you , you can decline their request . Then they should sort it out through Airbnb and should get refunded according to your cancelation policy.
Eh... the problem is that Airbnb may or may not ask to hear your side of the story first and just refund them partially or fully.
Maybe the best would be to contact Airbnb by yourself and briefly explain the situation : They arrived with a dog, stayed 24 h, didn't say anything to you or to the other guest so you were not avare of the problem and couldn't act timely to sort it out. They packed and left a mess behind them. Also tell Airbnb that you expect your cancelation policy is honored.
And then just wait. You may get a call from ABB representative trying to convince you to refund a guest. But you can refuse.