HI @Marisol125
I am not the most experienced host in the world, but to me this sounds like regular wear and tear and not something that would routinely be covered by the host guarantee. ( which is almost impossible to claim anyway so I hear)
Given that you yourself are not even sure if the problem was caused by the guest, or is just co incidental I personally would not expecting the guest to make any kind of payment ( even if the guest DID cause it you need to expect and anticipate a degree of wear and tear adn to me this sounds typical of just that. Not like the shower head had been ripped off the wall for example)
Re the repair time and use of the shower, I strongly feel communication is the key.
I would be talking directly to the guest/s, either in person or through airbnb messaging, explaining the situation, apologising for the inconvenience and asking if that timing would suit them without too much difficulty.
I personally would also be inclined to offer a bit of a sweetener for the inconvenience ( a box of chocs, a bottle of wine or some such - and I would also be guided by the guests reaction and communication).
some people may be VERY frustrated by not being able to shower at night, others may be not at all bothered.
Other hosts may offer different advice.
Good luck sorting it out!
Regards