Yes, you may answer Ament her brachot if you hear the whole bracha. Unless she is Notzri then you should not.
Explanation:
The Mishna in Brachot 8:8 writes we answer Amen after a Yisrael who says a bracha but we do not answer after a Kuti until we hear the whole bracha.
The Yerushalmi on the Mishna comments: A Non-Jew who blesses hashem we should say Amen afterwards. The Yerushalmi does not make a distinction between hearing the whole bracha or not.
The Rishon argue if a Goy and a Kuti have the same halachic status or not.
The Rosh says that a Kuti is more stringent than a Goy. Since the Mishna writes that one may only answer Amen after a Kuti fi he hears the whole bracha and the Yerushalmi contrasts that halacha with that of r a Goy which you my answer without hearing the whole bracha.
The Bet Yosef asks but the Gemara in Chullin 6a they decreed that the Kutim are considered regular Non-Jews. If that is true then if one hears a Kuti say a bracha he would have to answer Amen even if he did not hear the complete bracha? He answers Chazal only made the Kutim like Non-Jews to be stringent to lenient, therefore when the Kuti is more stringent then we may not answer until we hear the whole bracha. The reason we are more stringent by Kutim is that they were found to worship Avoda Zara and we don’t know who they are saying a bracha to. A Non-Jew won’t usually refer to Avoda Zara if they say a bracha to Hashem.
Talmidei Rabbeinu Yonah quotes an opinion that only by a Kuti we are permitted to answer Amen depending if we heard the whole bracha or not. A Non-Jew we don’t answer at all, unlike the Yerushalmi. We assume the Non-Jew is definitely thinking about Avoda Zara when they make a bracha. Therefore, nowadays we won’t answer after Kuti at all. Rabbeinu Yonah is of the opinion that there is no difference between them and a Non-Jew as well it depends if you hear his whole bracha or not.
The Rambam writes: Anyone who hears a Jew Shulchan Aruch bracha even if he didn’t hear the whole bracha from beginning to end and even if he is not obligated in the bracha himself , he is obligated to answer Amen. If a Goy, or a Min (a Kofer), or a Kuti or a child learning how to make brachot or if an adult who changed the wording of a bracha, made a bracha, do not answer Amen.
There are numerous understandings of the Rambam’s opinion of why we don’t answer after a Goy period.
Kesef Mishne has two explanations. One is the Rambam’s halacha depends on the case. It is not the same halacha for the whole list. A Goy, Min, or Kuti we do not answer Amen if you did not hear the whole bracha, but you may if you did. A child who is learning you do not answer at all even if you hear the whole bracha.
The second explanation is that the Rambam ruled like the Yerushalmi but he understood the Yerushalmi that a Goy is more stringent then a Kuti and you may not answer after him at all. This is like the first opinion in the Rabbeinu Yonah.
The Shulchan Aruch (215:2) quotes the Rambam verbatim. The fore according to the first understanding of the Rambam you may answer after a Goy if you hear the whole bracha. This is how the Gra understood d the Shulchan Aruch . According to the second explanation we don’t answer at all. So ruled the Bach.
The Rema writes: We answer Amen after a bracha of a Goy fi we heard the complete bracha. He is not arguing on the Shulchan Aruch but rather explaining. According to the Gra there is no argument between the Mechaber and the Rema. According to the Bach there is.
The Biur Halacha writes that even though there are those who understand the Rambam that you should not answer at all we can since there are Poskim who are of the opinion that even without hearing the whole bracha and you have to answer according the Yerushalmi , and so understood the Gra, it is enough to be stringent to say Amen after hearing the whole bracha.
The Yalkut Yosef. 215:2 differentiates between what religion the Non-Jew is. He writes if he is an atheist (even Jewish), or Notzri (Chr-n) then you should not answer Amen, even if you herd the whole bracha. However, if he is muslami then you may. You may also answer Amen to the bracha of a Mechallel Shabbat nowadays, but not forma Reform or Conservative Rabbi.
