The relationship between different minhgim and mesorot.
What would be the halacha if an Ashkenazi Jew visited a Yemenite Jew and was offered the Egyptin Desert Locust as a snack? Could he rely on their mesorah to permit him to eat what would otherwise be a Torah Prohibition?
Minhag and Psak din:
An Ashkenazi Jew who followed the psak of Rav Shlomo Zalman Auerbach z”l to forbid opening steel bottle tops on Shabbat sitting together with a Sefardi Jew who follows the psak of Rav Ovadia Yosef shlit’a who allowes opening such bottle tops. Could the Ashkenazi Jew ask his Sefardi friend to open the bottle for him?
1) An Ashkenazi, whose custom forbids him from eating locust, must refrain from eating it even when visiting his Yemenite friend. This ruling emerges from a Gemara wihch describes how Rav, who maintained that a certain type of food is prohibited (due to nat bar nat), did not fear to eat at the home of Shmuel (who permitted the food), because he knew that Shmuel would not feed him something that was prohibited to him. See Chulim 111b; Ritva, Sukkah 10b; Shach, Yoreh De’ah 119:20; Peri Chadash Orach Chaim 495:23.
2) There is a debate among poskim concerning this question. Although the Taz, and many poskim after him, permit somebody who has already accepted Shabbos to request from somebody who has not yet accepted Shabbos to perform a melachah for him, many make a distinction between this case and between the case of somebody who deems an action to be prohibited asking someone who permits it to do the action.
Therefore, in questions of issur and heter, one should avoid telling somebody who is matir to do something which one is stringent for, the more so when a possible Torah prohibition is involved. However, under extenuating circumstances there is room to be lenient (as the Minchas Shlomo (below) writes).
To go into the details of this halachah will require a full article, which we will please G-d write in the future. For sources, see Taz (263:3); Mishnah Berurah 63; Shulchan Aruch Harav 263:25; but for questions of prohibition see Taz 263:3, Mishnah Berurah 624:16; Magen Avraham 263:33. And see also: Minchas Shlomo, Vol. 2, no. 35, sec. 17.