There are visa requirements and exclusions for reasons. Being an enlisted soldier in a foreign army can’t, and shouldn’t, make you automatically exempt from them, simply because that country is an ally.
You’re assuming that something about being a soldier makes you above the law, or precludes them from having any quality that a foreign county would feel makes them ineligible for visa entry.
I’m not saying that America’s immigration system isn’t flawed, or that this one guys GF isn’t getting a raw deal. Just that you don’t know the details of this case, and you definitely shouldn’t assume that every soldier in a foreign military is someone who should automatically be granted visa entry simply because their country may have some sort of military alliance with your country.