Basically, two non-frum Jews come over to Moshe's house for Sukkos. They want to be nice, so they make a salad with lemon for him to eat. Except... not knowing what an Esrog is, they mistake it for a lemon, and it ends up in the salad.
What would halacha say in such a case? Are they responsible for the loss? And if so, how much would they owe?