@Vicdariz0 The proper procedure for your guest to follow would be to request a refund through Airbnb's Resolution Center. He may ask for $X and you will have the opportunity to accept or offer a different amount. If neither of you can reach an agreement on the amount of the refund within 72 hours, you or the guest can request that Airbnb get involved to settle the dispute. When that happens, a case manager will be assigned. He or she will review all the evidence and make a decision which will be final....no appeal.
In a situation where the guest found 2 rats, I would suggest that you reach an agreement with the guest as more than likely Airbnb would approve the guest's request.
Here's how the Resolution Center works:
https://www.airbnb.com/help/article/767/what-is-the-resolution-center