You are raising an interesting point! I would like to share a couple of ways of looking at the Gemara's discussion, according to which this issue might be alleviated somewhat.

First, it may sound far-fetched, but it is possible that the Nochim picked up on this garlic-eating custom of ours because they enjoyed it or found it beneficial.

Second, and perhaps more to the point, is that the Chidush being taught in our Mishnah is not in the Diyuk, that you are permitted to benefit from a Nochri who eats garlic, but rather the Chidush is in the second case which is stated, i.e. that you are not allowed to eat from a Kusi who eats garlic. The reason why this is a Chidush is because, l'Maskana, it is only a person who is eating garlic because of the decree of Ezra that is included in the Neder prohibition. Now, the Kusim only kept some Halachos meticulously -- typically those of the written Torah, but not of the oral Torah. Nevertheless, in the case of Ezra's enactment to eat garlic, they were careful to keep it.

Now, recall the Machlokes Tana'im regarding whether the conversion of the Kusim was valid, when it was based on the fear of lion attacks. If we go with the premise that the Kusim are legitimate converts, then they not only observe this garlic Takanah, but they are also are obligated to do so. That is why they are included in the Neder prohibition. So, according to this interpretation, the focus is not really on the admittedly unlikely case of a Nochri who eats garlic on Friday night, but rather on the case of a Kusi who does.

Warmest regards,

Yishai Rasowsky