People or Principles?

In Civ, we read a speech by Hannah Arendt, upon her reception of the Lessing Prize, in which she basically says that the hope of humanity lies in friendship, which consists of free and open dialogue between opposing points of view. Fine, nothing particularly radical there.
Then we learned to what degree Arendt lived this. Arendt, a Jew who had to flee Germany because of the Holocaust, maintained a close friendship with her mentor (and one-time lover) Heidegger, despite the fact that he went quite cheerfully over to the Nazis. When I heard this, I was appalled, but it does, I suppose, follow logically upon the question she rhetorically asks her audience: "Would any such doctrine, however convincingly proved, be worth the sacrifice of so much as a single friendship between two men?"
Well. Maybe if I were Chana, I would have a more literary reference here, but the connection that this idea made in my head was to Brothers In Arms, a sci-fi thriller with rather deep undertones written by Lois McMaster Bujold. In one scene, two characters discuss priorities:

"Surely it is more important to be loyal to a person than a principle."
"I suppose that should suprise me, coming from a Barrayaran. From a society that traditionally organizes itself by internal oaths of fealty instead of an external framework of abstract law-is that your father's politics showing?"
..."My mother's theology, actually... Her theory is that principles come and go, but that human souls are immortal and you should therefore throw your lot with the greater part. My mother tends to be extremely logical."

I had a conversation once with a friend about whether we would turn each other into the police for some crime. I said, "Yes. Absolutely. Without a doubt. I would feel sad, maybe, but I would do it," and she said, "No way." I'm not sure which of us was more shocked at the other. She kept turning to me and saying, "You would snitch on me? I can't believe you, you stool pigeon," and I kept turning to her and saying, "You would let me get away with murder? You would let a murderer walk away." (Yes, it was all in fun and we are still just as good friends. Nor have either of us been called upon to actually make this decision, thank G-d)
I think that our debate goes back to this same question of friendship versus principles. I believe quite strongly in principles, myself. I would, if I had the guts, turn my friend in. I am disgusted by Arendt's continuing friendship with a man who supported policies that included the extermination of her race. Now, fortunately for me, my principles include a great emphasis on respect for other human beings and so forth, or else I might be a very bad person.
This isn't a question of "love-the-sinner-hate-the-sin". I do that. I would probably not hate my friend if I turned her in for murder, but I would do it anyway. This is a question, quite simply, of whether one would sacrifice a friendship for a higher principle or vice versa.
And, like all big questions, both answers sound at the same time appalling and noble, depending on how you spin them. The question can be phrased, "Would you turn on a friend to fulfill your own ideology?" Or it can be phrased, "Would you continue to be friends with someone who you felt was evil?"
I know what my answer to that question is, but I have no idea whether it is right.