I am giving another ansewer, as you have edited your question to say that there are a greater proportion of Muslims in this non-Muslim country compared to the Muslim country, in this situation you describe. But I am keeping the other ansewer as it describes a situation that would mostly likely occur. Like my previous ansewer, there cannot be any (absolutely none) non-Muslims in this army, only Muslims. And I will quote the verse again so you do not have to refer to different answers:
Or think ye that ye shall be abandoned, as though God did not know those among you who strive with might and main, and take none for friends and protectors except God, His Apostle, and the (community of) Believers? But God is well- acquainted with (all) that ye do. (9:16)
Okay say the army is completely Muslims and you ar reling only on Allah and Muslims for help in the fight even though your country is non-Muslim. Now there is a situation described in the Quran where the hands of the Muslims were restrained by Allah from attacking the non-Muslims on account of there being some Muslims being present amongst the non-Muslims. Allah restrained there hands so the Muslims would not fight their fellow brothers:
And He it is Who hath withheld men's hands from you, and hath withheld your hands from them, in the valley of Mecca, after He had made you victors over them. Allah is Seer of what ye do. (48:24) These it was who disbelieved and debarred you from the Inviolable Place of Worship, and debarred the offering from reaching its goal. And if it had not been for believing men and believing women, whom ye know not - lest ye should tread them under foot and thus incur guilt for them unknowingly; that Allah might bring into His mercy whom He will - If (the believers and the disbelievers) had been clearly separated We verily had punished those of them who disbelieved with painful punishment. (48:25) When those who disbelieve had set up in their hearts zealotry, the zealotry of the Age of Ignorance, then Allah sent down His peace of reassurance upon His messenger and upon the believers and imposed on them the word of self-restraint, for they were worthy of it and meet for it. And Allah is Aware of all things. (48:26)
So in this situation the Messenger (pbuh) was given the command of self-restraint.
I must add that, how a non-Muslim country has a greater proportion of Muslims than a Muslim country is confusing since Muslims cannot migrate to a non-Muslim country, except for a brief business trip or Jihad or Dawah. The majority of Muslims do not come to non-Muslim countries for these reasons. This is part of the problem, it is better if the Muslims go to the own countries. As for the hypothetical person you are talking about, he clearly is not in the non-muslim country for a brief business trip, or Jihad (he is fighting but this is not Jihad as it is not for islam) or Dawah, so he should go back to his Muslim country of origin or any Muslim country.