Hi @Jorge314 - That's horrible to hear, and it's really unfortunate that you were put in this dillemma. Unfortunately I would have to agree with the other Airbnb hosts on this thread. It's almost impossible to prove that the guest put the "thing" in your toilet. But there are other ways to get your money back.
I would first like to clarify that if you make a deposit claim - I don't think that the review will be removed. Either way, $50 is a sizeable amount.
The odds are pretty slim, but I think it's worth a shot.
I would recommend starting by chatting with the guest, and getting them to admit that they dropped the "thing" in the toilet. Since the guest left you a nice review, I think it would be easier to get them to admit it. For instance, you can say something along the lines of
"Hi (Guest) I was doing a routine check of the listing, but I noticed that the flush is a little off. Did you notice any problems during your stay? It's no big deal, I'm sure that I can fix it myself. But I need to know if it's just a simple clogging or maybe some pipe damage"
If the guest admits that she threw something inside, then you can use it as evidence for Airbnb.
Alternatively, you can also just be frank with the guest. You can simply ask them if they dropped anything in the toilet that may have caused a clog. If they don't respond - bring in Airbnb. Usually if the guest does not respond withing 24 hours, Airbnb will make a decision on their behalf.
Hope this helps!