I may be a "newb" to this board, but I think you may be showing a lack of clarity on this issue. That or the OP showed a lack of clarity in stating what advice she wanted. I may be mistaken in these points but I don't see them yet. Perhaps it is because I am a "newb".
1) This is an ongoing situation? That doesn't improve your or the OP's position. The says that this has been festering and the heart-to-heart hasn't happened, when it should have, for a LONG time. I thought the heart-to-heart was imperative when this shiet had been going on for a SHORT time. The INFP NEEDS to STATE what is in her mind to the INFJ. You can call that "newb" thinking but I am fairly certain of this as a basic tennant of conflict resolution and management. IF this heart-to-heart HAS happened then I change my approach of course. But I asked this question very early on. I asked because in conflict resolution it shoudl be on your short list of questions to ask. The first being "are you in danger right now?"

Right after that I usually ask "Does this person know the things you just said to me?"
2) Well if the OP does possess the discernment to know what to "toss" I hope it is any advice of going J. Again, I am going with what the OP stated. First, in the title itself is "...come out unscathed". If you go J you will not come out unscathed from an INFJ, especially if you have to live with them for over half a year. That will be the longest several months ever. And INFPs do not do well in confrontations with INFJs where the INFP can't leave afterwards. INFJs are long-burn haters when they go bad. IF this person is as possibly immature as you say, this is a major concern.
3) I state this only as a generality: INFPs are NOT good at making decisions when their Fi is in overdrive. The first and subsequent posts of the OP lead me to believe this is the state she is in. This could be wrong of course. I don't know this INFP very well at all. But I think most third party observers would say that the OP is quite upset right now. IF this is the case, then another thing she should "toss" is any notion of doing anything with this first wave of emotions. Let it wash over and then make you choice.
4) The OP also made a brief referrence to this person being a friend. I'm assuming this has gone south. But how south? Again, this is like the third effing question you ask in conflict resolution. What can be saved? What do you WANT to be saved? Even if the answer is NOTHING, the person should still STATE that to herself and anyone else she is asking advice from. These are you navigation points in conflict resolution.