@Max-And-Kristie0 do you live nearby? Can you get over to
the house to let in the repair person? If so, schedule a time for the repair and inform the guests that an inspection will happen at xx time. Knock or ring doorbell at that time, if hey do not answer go in. If they do, state that you are there for their safety and will inspect the shower now and then do it. If they refuse, you can tell them to leave, as you think there is a safety issue and then immediately call Airbnb and ask for them to be rebooked elsewhere. You need to look at this now.
There may genuinely be cracks. Or, the guests could be looking for a free ride and have or will make cracks and then will contact Airbnb and say it is a safety issue and they want there money back. Yep, there are some really devious people.
Hope that it is an existing crack that is easily fixed and that these are nice people.