Buenos dias, paisana!
@Francesca290, what a mess. I feel for you.
Let the guest know what's going on and that they won't be able to use the shower. Tell them NOW.
Tell them you'd be happy to cancel the reservation (but help them find another place right away). Once you cancel, you'll be able to state a reason WHY you cancelled in the Reviews section.
You're going to take a hit from ABB for this unless you can prove the extenuating circumstance so get the plumber's estimate clearly stating that the shower is unusable and can't be fixed until such and such a date. Maybe ABB will accept this as an extenuating circumstance but your first priority is to find your guest a place for the price she was going to pay you.
I invited a few local hosts to my house for lunch the other day so that I could build a relationship with them as I know the day may come when I'll need them to step in for me in a situation like yours. And they may need ME so building these relationships is something that isn't discussed enough but should be.
Let us know how you make out.
Suerte!