I recently read, but now cannot recall the source, that an observant (a/k/a "Orthodox") Jew may eat at a less observant (a/k/a non-"Orthodox"> Jew's table. This decision came from an "Orthodox" rabbi.
An aside. Sefardi and Mizrachi Jews don't divide Jews into European-defined camps of "Orthodox," "Conservative/Traditional," and "Reform/Liberal" or any other segregationist grouping (Reconstructionist, Humanistic, etc. and et al). To a Sefardi, all Jews are Jews - some simply are more observant and others less observant.
While I was pleased to know I could eat at the table of someone who normally ignores kashrut - assuming the host/hostess presented me with kosher food (catered by a kosher caterer or cooked in/with new single-use utensils and served on single-use plates, etc.) - I also realized that this revelation coming in 2012 was, in fact, nothing new.
Hakham Yosef Messas long-ago (c 1924) ruled this way.
From Ideals, Institute for Jewish Ideas and Ideals (http://www.jewishideas.org/articles/rabbi-joseph-messas)
An example of this is seen in (Hakham Yosef) Messas' experience in Tlemcen. He arrived in the city in 1924 and found that although there was proper shehitah, the kosher butcher shops were all open on the Sabbath. At this time, there wasn't yet a system of mashgihim who would testify to the kashrut of an establishment. Instead, all of Morocco followed the old approach of relying on the personal religious observance of the butchers. This practice was based on the assumption that if you could eat in someone's house without questioning if the food was kosher, you could also purchase from his shop. Yet this principle only applies to observant Jews, and in this case the butchers were all public Sabbath violators. According to Jewish law, these people simply did not have the religious credibility that observant Jews need from their butchers.
At first glance, there appears to be no avoiding the conclusion that since the butchers were not religiously reliable, observant Jews were obligated to give up meat. (As Messas explains, it proved impossible to open a shomer Shabbat store to sell the meat.) Yet was this the only possible conclusion? Messas recognized the many problems that would arise if he declared the butchers not kosher, not least of which would be that many people would simply ignore his declaration, thus destroying any communal standards of kashrut observance. He was also concerned for the honor of his community, which was, as he tells us, being portrayed as a place where everyone ate non-kosher. He therefore offered a radical halakhic justification for the status quo. He argued that since, according to one approach in the medieval authorities, the butchers were not violating any biblical commands which in Temple days would be regarded as a capital offense, they could still be regarded as trustworthy with regard to the meat they prepared and sold. He also offered other reasons why the local butchers, despite being Sabbath violators, could be believed in matters of kashrut. Messas surely knew that he was going out on a limb with this ruling, but under the circumstances he believed that it was the only proper halakhic answer, one that dealt with the reality he was confronted with (Mayim Hayyim 1:143).
While in earlier times it was obvious that one must avoid patronizing non-shomer Shabbat butchers, Messas felt that in his era, when so many were not observant, it was important to find a leniency. This is just one of many examples where Messas shows how dynamic halakhic decision-making can be, and how it can lead to some surprising conclusions. In this particular case it was very hard for those outside of his community to agree with his conclusions. Yet as R. Nathan Neta Leiter wrote to Messas, after expressing his disagreement: "I can find one justification for you, and that is what our Sages said, ‘Don't judge your fellow until you are in his place,' and I do not know the nature of your country" (Tziyun le-Nefesh Hayah, no. 29).
This trend of Messas is seen in other responsa as well. His most famous halakhic ruling is that in an era when women generally go about with uncovered hair, it is no longer regarded as nakedness. As such, it is entirely permissible today for married women not to cover their hair (Otzar ha-Mikhtavim, vol. 3, no. 1884, Mayim Hayyim, vol. 2, Orah Hayyim no. 110). He defended this opinion at length, and a well-known Moroccan halakhist from the subsequent generation, R. Moshe Malka, later chief rabbi of Petah Tikvah, expressed complete agreement with Messas' view (Ve-Heshiv Moshe, nos. 33-34).
The approach of limud zekhut, that is, of finding justification for the practices of the masses, has a long history in Judaism. It is this approach that Messas adopts in his responsa on women uncovering their hair. Since, as he tells us, the wives of pious people do this, there was a great motivation to find it halakhically permissible.
As Shlomo allegedly said אין שם חדש תחת השמש
