Hello, @Koberheu! Wow, this is a really good question. If I am understanding correctly, you are asking where the responsibility of things such as those mentioned in Isaiah lie–with the church or with government.
The passage of Isaiah that was pointed to in the conversation comes after a section of the book (chapters 40-55) in which Yahweh is promising hope of restoration to Israel and the other nations based on grace, not merit. But in chapters 56-66, God speaks through the prophet to say that even though He will save by grace, He still calls His people to live righteously. This section speaks of the fact that God’s true people are not people of one physical lineage or even nation, but that He will have true worshipers in other nations as well. So now the focus is taken off of theocracy and put on the true people and individual worshipers of Yahweh. So here we see God is already speaking of not a nation governed by some covenant law such as Moses’, but people from all the nations worshiping God in how they live, in how they deal with others, etc. So even in this passage of Isaiah in which we find the verse pointed to in your husband’s conversation with your friend, we see that the point of the passage as a whole is to say something like, “Look, I called you and chose you as a nation to be my light to the world. I gave you my law and made a covenant with you. But you broke my covenant. You brought me sacrifices and feasts while you were living in a way that goes against Me. You did not clothe those who needed clothed and you did not feed those who were hungry. You did not perform justice or extend mercy. So now you have had judgment come upon you. But, my people, there is hope. There will be restoration, but my people will no longer be of one ethnicity and nation. My true worshipers will be from among all the nations, and they will worship me in word and in deed. While you wait for this restoration, I also call you to live righteously and to not be like you were before, ignoring and even at times despising those who were in need, even those of your own flesh and blood.”
Israel had been a theocracy, and so their government was indeed responsible for doing or neglecting to do those things win the covneant which made provision for how to treat others. However, under the new covenant, we as believers, the Church, are responsible for doing these things, though government has a role in it by performing justice. Looking at this in the New Testament, the government and the church have distinctive, yet sometimes overlapping roles. The government’s job is to punish evil (Romans 13:3-4 and 1 Peter 2:14). The church’s job is to spread the message of the gospel in word and deed, thereby witnessing God’s love to people–shining the light of Christ in a broken and hurting world. However, in, for instance, the United States, where citizens have means to affect government through voting, that duly falls under the stewardship of Christ’s disciples. It is a responsibility we have to act for the good of others, to do what we can for others. So, although government can and perhaps should be used as a means of helping those less fortunate, as the Church we also need to be actively reaching out to help those in need and should not assign that responsibility to government, because we are the ones who are called to show God’s love and light to the world for the sake of winning people over with the gospel, both in how we preach it and in how we live it.
I hope this helps. Let me know your thoughts so far