Question: I invited a guest for a Shabbat meal and he told me that he only eats from a particular Kosher certification of meat. Do I need to buy that type of meat or can I buy meat with the Kosher certification that I usually use?
Answer: There are two possibilities: A. Buy what he wants. There is a guest, so go out to greet him. B. You can tell him, "No, I always buy this Kosher certification." Does a guest dictate what you do in your house? I once heard a story about a Rabbi in North Africa who had an event at his house, a bar mitzvah or something, and he honored one of his guests, who was also a Rabbi, to lead the "Birkat Ha-Mazon". The guest replied, "Thank you, but I do not eat from this Kosher certification." When he heard this, the host took a key, locked the door, stood next to him with a chair and said: "You will eat this right now or I will break this chair over your head." "Kol Ha-Kavod" – Way to go! What is this? You are invited by people, and you say that what they are eating is not Kosher?! If this is how you feel then don't come, or say that you have a stomach ache or that you don't like the food. You are not obligated to eat everything, but don't come to someone's house or event and say it is not Kosher enough.
One Sukkot, students came to visit the Sukkah of Ha-Rav Shlomo Zalman Auerbach. He served them fruit. They took the fruit and separated "Terumot and Ma'asrot" (tithes) right under his nose. He took the so-called "Terumot and Ma'asrot" that they separated, ate them and said: "Bal Tashchit" (It is forbidden to wantonly waste things). What nerve! You are invited to a Rabbi's house and you say that "Terumot and Ma'asrot" have not been separated from his food?! My father-in-law z"l once told me that he visited Ha-Rav Eliyahu Dessler on Pesach. The Rav put out oranges and my father-in-law ate them. Rav Dessler said: "You are the first person who ever ate the oranges I put out" (others feared that the ink of the seal was Chametz – leaven).
One who wants to act strictly may do so. May a blessing come to anyone who is strict. The Talmud Yerushalami quoted by the Tosafot in Avodah Zarah (36a) says, however, that one of the conditions of one who is strict is that he does not shame other people. Being strict is praiseworthy, but shaming others is forbidden. There are many people who are strict in their home, but they eat what is served when they are guests; obviously, provided that the food is Kosher. It says in the Book of Tehillim (101:2), "I walk with wholeness of heart within the confines of my house" – in the confines of my house I am strict, with other people I am not. In the book "Ve-Alehu Lo Yibol" (vol. 2, p. 66), which contains stories about Ha-Rav Shlomo Zalman Auerbach, Ha-Rav was asked by female seminary students about eating at certain families' homes who were not as strict as they were. He said: "I do not understand what you are asking. Will they serve you non-Kosher food there?" "No." He responded: "Then eat." They asked him: "Ha-Rav also acts this way?" He said: "Yes. When I am invited to a wedding, I eat what is there. What I do not eat in my home, I eat when I am invited." Someone who is invited and feels in his soul that he must be strict should not go.
The Shulchan Aruch (Yoreh De'ah 112:13) says that some people who live outside of Israel eat commercially baked bread which is produced by non-Jews (called "Pat Palter"). There are others who are strict and only eat bread baked by Jews. If a host does not eat "Pat Palter" and his guest does, and the guest wants to eat "Pat Palter" because it tastes better, the Shulchan Aruch rules that the host should say the blessing on the "Pat Palter" and eat it in order to honor his guest. This means that the host should give up a stricture for the sake of the good feeling of the guest. And all the more so, a guest should not place his strictures upon his host.
This is from the side of the guest. From the side of the host, if you are willing, give the guest what he wants. He is ready to eat from your utensils, so he certainly does not think your food is not Kosher.