We went away for Shabbos and let some friends stay in our apartment without charge and let them use our dishes, pots etc. The dishes and pots were all clearly marked whether they were milchig, pareve or fleishig. By mistake, they used a Teflon coated pareve frying pan and made it fleishig. I want to have a pareve frying pan since sometimes I’ll fry an egg and someone will want to eat it on a cheese sandwich, which one can’t do according to many poskim, if the egg is fried in a fleishig frying pan. Can I ask our guests to give me a new frying pan since one can’t kasher the frying pan?
In order to address your question we have to first clarify what was the relationship between your guests and your frying pan since that will determine where to find your answer.
Since your guests used your home and its furnishings for free, your guests had the legal status of a sho’eil-one who borrows an object. The moment she used your frying pan she was borrowing the frying pan from you. We note that a sho’eil is one of the four types of shomrim which are described in the Torah (Shemos 22, 13).
The next issue is how to classify what your guests did to your frying pan. Your guests damaged your frying pan since its use became limited. However, the frying pan was not damaged in a physical sense. Outwardly, the frying pan is the same as it always was. It is only because of the halacha that there is any damage. Damages which are not noticeable are called by the Gemara: hezeik she’eino nikar. The Gemoro (Gititin 52-3) rules that one is only liable for this type of damage if it was done intentionally. If it was done without intention to damage, one is not liable for such damages. For example, the Mishna writes that if one intentionally rendered a kohain’s terumo to be tomai (ritually impure), requiring that it be burned and not eaten, he is only liable if he acted intentionally. If his act was unintentional he is not liable.
The next issue that is relevant is whether the lenience granted for undiscernible damages applies to a shomeir. The reason it may not is that a shomeir has a responsibility to return the object that was entrusted to him in the same state as he received it. Therefore, perhaps even though there is no physical damage, nevertheless since the object had a difference in status from what it had when it was received, perhaps a shomeir is liable. In fact that is the position of the Maharshal (Yam Shel Shlomo: Bava Kama 9, 20). However, the Shach (363, 7) vehemently objects and writes that this comment of the Maharshal is merely an oversight. The opinion of the Shach is accepted.
Thus we have to consider whether the act of your guest is classified as hezeik sheino nikar or not.
The Sha’ar Hamelech (Choveil Umazik 7, 3) was asked a very similar question. In his situation, a lady borrowed a fleishig pot and by mistake stuck in a milchig spoon rendering both her food and the pot treife. He agrees with the Shach that a shomeir is not liable for undiscernable damages and he rules that the borrower is consequently not liable to pay for the pot.
The Chassam Sofer (Gittin 52B) cites the Sha’ar Hamelech and disagrees. He says that if the spoon will give a milchig taste to the pot to the extent that when one subsequently cooks in the pot, the food will have this milchig taste, then the damage is noticeable since taste is noticeable. He remains with a doubt in case we do not permit further use of the pot because we are uncertain if it will give off a milchig taste, since it is possible that one who borrowed is liable even in this case since at the end of the day the pot he returned is no longer usable.
Likewise, Rav Shlomo Zalman Auerbach (Minchas Shlomo 1, 88) disagrees with the Sha’ar Hamelech because he also maintains that if one can taste something, it is called noticeable. Therefore, he rules that if the milchig taste is present in the fleishig food the damage is considered noticeable. He maintains further that even if the taste is not present in the food but just in the spoon according to many Rishonim the borrower would be liable.
Similar to these poskim, the Pischei Teshuvo (385, 1) cites a Pri Megadim that if one stuck a treife spoon into someone else’s hot kosher food rendering the food treife, sometimes he will be liable and sometimes not. If the taste of the treife food that was absorbed from the spoon is the same as the taste of the kosher food, then even though the kosher food became treife he is not liable since the treife taste is not noticeable. However, if the treife food has a different taste from the taste of the kosher food then the one who stuck in the spoon is liable. The reason is that taste is noticeable and therefore, we are no longer dealing with hezeik sheino nikar.
The Kesav Sofer (Choshen Mishpot 26) and Har Hatsvi (Choshen Mishpot 385) qualify that the Pri Megadim is confined to the case where the food tasted differently but since a pot doesn’t have taste, the taste is always unnoticeable. However, it should be noted that this is against the Chassam Sofer cited above who says it suffices if the taste will be noticeable in food that is subsequently cooked in the pot.
Even in case the taste is noticeable, one has to consider what damage was done. If one damages something in a way that it is only a chumro (stringency) not to use it, then there is a dispute (See Shach 306, 10) and Ketsos (306, 10)) if one is liable for the damages. If one fried an egg in that frying pan then it is a dispute whether he can subsequently eat the egg on a cheese sandwich. However, you correctly state in your question that according to many poskim you can no longer fry an egg in the frying pan if you intend to put the fried egg in a cheese sandwich so there is definitely damage that has been done. We should also note that the previously cited Chassam Sofer writes that it is possible that everyone agrees that a borrower would be liable even in case the only reason the object cannot be used is based on stringency.
In conclusion: It certainly would be proper for the guest to replace the frying pan and keep the fleishig frying pan for herself.