If a Sepharadi hears an Ashkenazi recite a Beracha in a situation where Sepharadim do not recite a Beracha, should he answer "Amen" to that Beracha? For example, Ashkenazim have the practice to recite a Beracha before Hallel on Rosh Hodesh. Sepharadim do not follow this custom, and thus a Sepharadi who recites the Beracha in this situation would be considered as having recited a "Beracha Le’batala" (a Beracha in vain). Similarly, Ashkenazim customarily recite a Beracha before they put on their Tefillin Shel Rosh – "Al Misvat Tefillin" – in addition to the Beracha recited before laying the Tefillin Shel Yad. (They then add "Baruch Shem Kebod Malchuto Le’olam Va’ed" after placing the Tefillin Shel Rosh.) Here, too, it would be forbidden for a Sepharadi to recite such a Beracha, even though Ashkenazim recite it.
Another example is a situation of a woman who performs a Misva from which women are exempt ("Misvot Aseh She’ha’zman Gerama"). Sepharadic women may not recite a Beracha in this case, whereas Ashkenazic women indeed recite the Beracha in such situations, such as before shaking a Lulab.
In all these cases, if a Sepharadi hears the Ashkenazi recite the Beracha, should he answer "Amen"?
Rabbi Moshe Halevi (Israel, 1961-2001), in his work Birkat Hashem, rules that a Sepharadi may answer "Amen" to such a Beracha. Since the custom among Ashkenazim is to recite this Beracha, and the Ashkenazi thus does not act incorrectly by reciting it, the Beracha is considered valid and legitimate. As such, it warrants the response of "Amen," even by somebody who does not follow that custom.
Hacham Ovadia Yosef, however, in his work Yabia Omer, disagrees. He notes that the Halachic principle of "Safek Berachot Le’hakel," which states that one does not recite a Beracha when its requirement is subject to doubt, applies to answering "Amen," as well. Meaning, one does not answer "Amen" to a Beracha if there is some question as to whether this response is warranted. Therefore, in the cases mentioned above, where Ashkenazim and Sepharadim disagree as to whether the Beracha is required, one should not answer "Amen" to the Beracha. Halacha indeed follows this position, and therefore a Sepharadi should not answer "Amen" to a Beracha recited by an Ashkenazi that Sepharadim are not accustomed to reciting. He can answer "Amen" silently in his mind, but he should not answer "Amen" verbally.
Summary: If a Sepharadi hears an Ashkenazi recite a Beracha that Sepharadim do not recite, such as the Beracha before Hallel on Rosh Hodesh, the Sepharadi should not answer "Amen."
