This became a long answer. I understand very well that you don't want to open up too much details here. But let's give it a try, should we?

You attest both sides incompetence. Which is so far fine. If you want to help, the best thing from my point of view would be to make it clear to both sides that they have the potential abilities to resolve the situation to their satisfaction. And that they haven't found them yet. What could that be? Because, after all, you cannot judge their potentials, can you? There must be room for surprise and change of perspectives.

The case of cracking could actually be with yourself.
If you don't succeed to deliver a message as well as not producing curious questions within those involved ones, your conclusion is so far that your inability to do so justifies undiminished anger. I think this is "too short a jump". I would rather say it justifies a completely un-angry friend. Maybe someone who does not give the case lost. But maybe who does and give it to others.

The fact that you doubt your conclusion is a very good sign. It means that you still have legitimate uncertainty as to whether you might not achieve a higher vibration in your interaction. I would like to encourage you in this and tell you that there can be a complete reversal of your attitude towards this issue.

For is it not indeed possible that you yourself - as well - have not yet found a wise way to cope with such a situation? Which is a thought you obviously have.

I do take responsibility for it, I do not identify with it, but I do willingly carry it as a possession because I have yet to justify its relinquishment in my own mind.

Oh, that's a very interesting formulation that I would recommend investigating further. What could be a very good reason for the renunciation? And what do you feel if you only renounce for a moment?
Is it not the case that we find it very difficult to renounce our anger in general? As if anger is not also like a well known companion who has been used to us for a long time were leaving us?

Yes, I do ask you of the anger being a "possession" of yours. What if anger was something that wasn't true? What if anger is an illusion, though strongly felt, but stems from the fact that we have a sense of "I". But this sense that there is an "I" is not best representing reality, it is strongly trying to cling to some thing that it thinks is real, like "anger" (and other feelings).

This is seen when we suddenly become terribly angry at strangers we have never met before. Someone comes into a café and his sheer presence upsets us terribly. But is it really the person who upsets "me" or does anything in this persons appearance re-mind me of past upsetting experiences? The way he talks, how loud he talks, how he moves or gestures in a certain way, how he stands or even smells and so on. This person did not even look at me or talk to me but I feel I truly dislike him.

And yet, if a person does not listen to quiet reason, is not anger a necessary tool? And if it is morally necessary, then it is appropriate to express it unmitigated, isn't it?

If the reason is not only quiet on the outside but stays quiet in the inside, there is chance it might be heard. Otherwise: No, I don't think so. It needs sometimes though a really strong expression and even "as if anger is there" but used only as a feigned element to get another ones attention but stems from truly wanting another one to be well. Like in some zen stories vividly narrated. Anger is actually a weak tool, irritation is much better
;-)

When you imagine a bandwidth where people navigate and gain insight, you can well imagine that you and the other two are on different points. It's very difficult to bring someone exactly to your point if you haven't received a mandate for it. So if you want to settle a conflict, it is absolutely necessary that you receive the mandate not only from one side, but also from the other.

If you want the perpetrator to cease to be a perpetrator and you are not really concerned from your heart to want to receive the mandate from him as well, your efforts will probably be in vain. For then one could assume that you are not really interested in a solution to the conflict, but that you are interested in the other person ceasing to behave like an immoral idiot. The victim should continue to have a part in involving other people, from whom she can more easily assume, for example, that they would receive a mandate from the perpetrator to resolve the conflict. If the perpetrator does not accept such a mandate from anyone and this is a life-threatening situation, the state executive is the address to call in such cases.

One can only be de-escalating if one is accepted as a de-escalator.

Therefore, I asked whether it was a threat to life and limb, because then I would call in the police or the court. But as long as there is a chance of a mediation (not meditation!) process, a person accepted by both sides should accompany such a process. In such serious cases I would advise your acquaintance to turn to a professional.

P.S. Maybe this and this article of mine can also be of service to you.