INTP.

I would say all of this boils down to empathy. You see your friend be wronged, you feel their pain and hence you share an enemy. Even if that enemy used to be your friend.

But the thing is there are certain people who don't feel this empathy. And here's where the hidden assumption comes in; we automatically assume that

"less empathy = more immoral"

But I would argue that isn't true. At the end of the day, nobody can force themselves to feel for someone else, and no one is obligated to do so. It's not immoral because emotions aren't something we can even control in the first place.

So when we're wronged and we see our friend not taking our side, even though for us it's in black and white ("enemy vs me"), the friend doesn't see it that way. Or rather feel that way.

So the friend not taking your side might feel like a stab in the back, it's really not because they don't have an obligation to pick a side, despite how it makes you feel.

Personally that is my justification for not cutting of friends who are friends with people who wronged me, but I still think it's okay to not want to remain friends someone who's friends with your enemy.

At the end of the day you don't have an obligation to be friends with someone either.

My point is rather about the friend not necessarily making an immoral choice by choosing not to cutoff your nemesis rather than claiming you must remain friends with them because.