Q. Someone who is careful not to recite a brocho unless there is someone present to listen to it and answer amen, would the amen responded by a Gentile who understands or is explained before the meaning of the blessing count as a proper amen?
A. Borchi Nafshi (Devarim p. 188) mentions a similar question on someone who wanted to take a drink while traveling in a taxi. He wanted to know if the amen answered by the understanding Arab driver would count. Horav Y. Zilberstein Shlit’a maintains it would, since Rambam (H. Sanhedrin 10: 10) rules we do not suppress or inhibit a Gentile from performing a mitzva, even when done only for receiving reward. Being that answering amen implies emunah and belief in Hashem, it constitutes the compliance of one of the Seven Noachide mitzvos and should be promoted. In addition, there was no restrictive acceptance as to who may answer amen, when the commitment was taken.
However, Horav Shlomo Miller Shlit’a disagrees and maintains that the amen pronounced by a Gentile, even when he understands the meaning of the brocho, is in this case of no value.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a