There is a debate among the Halachic authorities regarding the case of somebody who hears another person recite a Beracha which is unwarranted according to the custom of the listener. A classic example would be a Sepharadi who hears an Ashkenazi recite a Beracha over the Hallel reading on Rosh Hodesh. Sephardic custom is to recite Hallel without a Beracha on Rosh Hodesh, whereas Ashkenazim recite the Beracha of "Likro Et Ha’hallel" before Hallel on Rosh Hodesh. If a Sepharadi would recite the Beracha, he would be in violation of reciting a Beracha Le’batala (blessing in vain), whereas an Ashkenazi is obligated to recite this Beracha according to his tradition. The question thus becomes whether the Sepharadi should recite "Amen" over such a Beracha. On the one hand, we might argue that since this Beracha is invalid according to Sephardic custom, the Sepharadi should not answer "Amen," since one should not answer "Amen" to a Beracha Le’batala. On the other hand, since the person reciting the Beracha is allowed and even required to do so, perhaps the Beracha is considered valid and thus warrants the response of "Amen" even by those who follow a different custom.
Another example of this issue is the case of a woman who performs a Misvat Aseh She’hazman Gerama – a "time-bound" Misva, from which women are exempt. The custom among Sepharadic women, based on the ruling of the Shulhan Aruch, is not to recite a Beracha over a Misva which they perform voluntarily. After all, they cannot say "Ve’sivanu" ("He commanded us") concerning such a Misva. Thus, for example, if a Sephardic woman shakes the Lulab, she does not recite the Beracha of "Al Netilat Lulab." Ashkenezi women, by contrast, have the practice of reciting a Beracha in such a case. The question thus arises as to whether a Sepharadi who hears an Ashkenazi woman recite a Beracha over the Lulab (or another Misva from which she is exempt) should answer "Amen" to her Beracha.
Similarly, Ashkenazim have the custom to recite the Beracha of "Al Misvat Tefillin" before putting on the Tefillin Shel Rosh. Sepharadim, of course, do not recite this Beracha; we recite only the Beracha of "Le’hani’ah Tefillin" which covers both the Shel Yad and the Shel Rosh. Thus, the question arises as to whether a Sepharadi answers "Amen" when he hears an Ashkenazi recite the Beracha of "Al Misvat Tefillin" over the Shel Rosh.
Rabbi Moshe Halevi (Israel, 1961-2001), in his work Birkat Hashem (vol. 1, p. 254), poses the theory that as long as the person who recites the Beracha has a legitimate basis for doing so, one should answer "Amen" to the Beracha. Since the person recites the Beracha on the basis of a Halachic ruling by one of the Poskim, and he is legitimately following his custom, the Beracha is considered valid, and one should answer "Amen" even if his custom is not to recite the Beracha. Rabbi Moshe Halevi compares this situation to the case of a Sephardic woman who hears a man recite the Beracha over the Lulab, or over Tefillin. Clearly, she would not be allowed to recite these Berachot, but she would nevertheless answer "Amen" if she hears a man recite them. Similarly, if a person recites a Beracha that is required according to his tradition, the listener answers "Amen" even if for him the Beracha would constitute a Beracha Le’batala.
Rabbi Moshe Halevi cites in this context the ruling of the Be’ur Halacha (by the Hafetz Haim, Rav Yisrael Meir Kagan of Radin, 1839-1933) that even if one recited a Beracha which he should not have recited, those who hear the Beracha should answer "Amen" if there is at least one Posek who requires reciting the Beracha. (The Be’ur Halacha mentions this ruling in 215:4.) Although Rabbi Moshe Halevi does not go as far as the Be’ur Halacha, he notes that certainly if one legitimately recites a Beracha required according to his Halachic tradition, those who hear the Beracha answer "Amen" even if they follow a different tradition.
Hacham Ovadia Yosef, however, in several places (including Yabia Omer, vol. 1, 29:9), disagrees. He writes that since Sepharadim follow the rulings of Maran regarding the recitation of a Beracha in the aforementioned cases, Sephardic practice regards these Berachot as Berachot Le’batala. As such, Sepharadim should not answer "Amen" to these Berachot. Since a Beracha in these cases is considered according to Sephardic custom as being recited in vain, Sepharadim should not answer "Amen."
One should follow Hacham Ovadia’s ruling in this regard, and thus if a Sepharadi hears an Ashkenazi recite a Beracha that is not warranted according to Sephardic custom, he should not answer "Amen."
Summary: If a Sepharadi hears an Ashkenazi recite a Beracha that is not warranted according to Sephardic custom, he should not answer "Amen." Common examples of this Halacha include the Beracha recited by Ashkenazim over Hallel on Rosh Hodesh, an Ashkenazi woman who recites a Beracha over a Misva from which she is exempt (such as Lulab or Succa), and an Ashkenazi who recites the Beracha of "Al Misvat Tefillin" over the Tefillin Shel Rosh. A Sepharadi who hears an Ashkenazi recite a Beracha in these cases should not answer "Amen," since the Beracha is not warranted according to Sephardic practice.
