Your friends are simply technocrats.
Your friends are, in a nutshell, trying to assert that there is a list of things (which your friends write) which are "absolute", that your friends' list is unarguable, decisive, platonic.
Obviously, that concept is total nonsense.
It's just a list they have come up with that they approve of, nothing more or less.
To counter their argument (or, let's put it this way, to show them how silly their position is), simply list obvious cases where it is by no means clear or "obvious" if harm is being done or not.
A simple example - in my current 21st century western-world life there is little chance of me being involved in a war or even a fist-fight. What is literally the worst harm that is done to me from day to day my whole adult lifetime? One group of humans steals about half of all my money every year - a tremendous harm. Your friends would babble "oh that doesn't count - it's not on our list." Another instant example is killing people via abortions, again they would babble "oh that doesn't count - it's not on our list." Another instant example is being arrested / imprisoned by other humans who happen to have on some "blue clothes" - and once again your friends would babble "oh that doesn't count - it's not on our list." Another instant and topical example in this historical era is a group of humans emitting pollution (greenhouse gases and so on) which indeed causes vast harm to other humans, and again your friends would babble "oh that doesn't count - it's not on our list."
Via examples like this, you can decisively show them that they do not have some sort of indisputable mathematical list, it's just ... a list they made up. (Of course they'll just ignore you ... and, hand you their list again, stating that the list is the list.)
Each case listed is highly and hotly debatable; the notion that there is an "absolute" answer is absurdist.
In a word your friends are technocrats politically - folks who think they "know best and absolutely", who think they are experts and that experts should "rule the world" as it were; note they are literally stating "x doesn't need to be debated because, we've given you the list, experts (like us) decide the list and there is no debate".