When does one stand for Kaddish?
The Rama (O.H. 56:1) writes that one should stand whenever one hears Kaddish being recited, and indeed, this is the Ashkenazic custom. On the other hand, the Mishna Berura (M.B., O.H. 56:8), citing the Sha’ar Hakavanot, says that the custom of the Arizal was to sit during Kaddish, unless he was already standing when Kaddish started to be recited, or in the Kaddish that follows the Amida. If one is already standing when Kaddish is being recited, it is considered disrespectful to sit down. Similarly, if one is standing and knows that Kaddish is about to be recited, such as immediately after the Amida, one should not sit down quickly to avoid standing during Kaddish, as this too is disrespectful. Rather, if one is already sitting, does not have to stand up specifically for the recital of Kaddish. The common Sephardic custom is in line with the Arizal’s practice.
Regarding a Sephardic person who happens to be praying among Ashkenazim, Rabbi Ben Zion Abba Shaul (Or Lezion, vol. II, ch. 5, § 9) says that one should not deviate from the local custom and should stand during Kaddish. If, however, there are people in such a communtiy who sit during Kaddish, such as the elderly or infirm, and it will not be noticeable that one sits, one may sit down during Kaddish. Rabbi Ovadia Yosef (Yehave Da’at, vol. III, § 4) concurs with this position
Regarding the Kaddish which is recited before Barechu on Friday night, the Sha’are Teshuva, citing the Mahari Saruk, says that one should stand for this Kaddish. The Be’er Hetev, understands this to actually mean that one should stand during the Kaddish recited before the Amida, when the words “Ufros Alenu” are recited. It is quoted in the name of Rabbi Haim Vital that one should stand for the Kaddish before the Amida of Shabbat night because it is at this time that one acquires an additional soul. Although the source to this is not clear, in any event, the Ben Ish Hai (Vayera) writes that it is during the Kaddish preceding Barechu that one acquires the additional soul and one should therefore stand then.
Notwithstanding, the Moroccan custom is to sit during the Kaddish which precedes Barechu on Friday night as the source to stand during this Kaddish is not written in the original works of the Arizal and they even seem to imply the opposite, specifically that one should remain sitting for all of the Kaddish’s. Furthermore, the Kaf Hahaim (K.H., O.H. 56:53) points out that if one stands during this Kaddish because one is accepting an additional soul, then one should stand during the recital of Nishmat on Shabbat morning, when one again acquires an additional soul, and it is known that this is not the common practice. As such, it would be illogical to stand for one part of the prayer to accept the additional soul but not the other.
Summary: The Moroccan custom is to remain seated when Kaddish is recited before Barechu on Shabbat night. Regarding this and all other recitals of Kaddish, one should not sit if one is already standing up. If one is standing, one should not quickly sit down to avoid standing up when one knows Kaddish is about to be recited.