The Chelkas Yoav in his discussion of esrog hamurkav (O.C.32:6 d”h yatzasi) uses the same gemara a bit differently. What if a ben noach grafted together a lemon tree and an esrog tree – can that fruit be used for the mitzvah of esrog? Lichorah it is a machlokes haposkim between the Levush and Shach. The Levush paskens like the view in Chazal (which seems to be well supported in Rishonim) that even a ben noach is not allowed to graft together different species of trees. Therefore, an esrog hamurkav is disqualified because “ne’evda bo aveira,” something that comes about through an issur is disqualified for mitzvah use. The Shach, disagrees and holds that there is no issur of grafting for a ben noach, so such an esrog would seem to be acceptable.
Not so fast says the Chelkas Yoav. We see from Brachos 53 that even if something is not assur for a ben noach, since the same act is assur for a yisrael, it is considered ne’evda bo aveira and is disqualified from mitzvah use. Even if the Shach is right that a ben noach is allowed to graft an esrog and lemon tree together, since a yisrael cannot, the fruit cannot be used for a mitzvah.
According to the C.Y. the reason a candle lit on Shabbos by a ben noach cannot be used for havdalah is not a din in hilchos Shabbos because Shabbos is an issur cheftza. It’s a din in ne’evda bo aveira – anything which if done by a yisrael would be an issur cannot be used for a mitzvah purpose even if the act was done by a ben noach.