I might be biased by my type, but I appreciate the OP thoughts for sure.
I tend to think that every 6 contains both phobia and counterphobia, and I assume how much of each we think in and behave with is a complex mixture of genetics, environment, and development. I assume that each 6 has a baseline tendency of where we tend to lie on the P-CP spectrum, but I would be willing to bet that most 6s go through fluctuations on that scale during their lives as they grow and change.
I think that one of the hardest parts of being a 6 is weighing the appropriateness of reaction - sometimes I find it very difficult to figure out what a middle ground between phobic response and counterphobic response would be - the key, I believe, is that it lies on a different axis, one of action versus non-action. I think one of the most difficult but healthiest things we learn as 6s is how to not respond (perhaps the reason 6 integrates to 9), which opens the door to understanding that response itself is a choice, and that we can modulate how much we choose to respond, rather than just whether we choose to fight or flee in proportion to the perceived threat.
To me, seeing a phobic or counterphobic response out of someone seems basically the same in terms of level of health, as in the level of (un)health corresponds with the intensity of response. So, as for your friend who shifted from phobic to counterphobic, I do wonder what may have caused that shift (I assume some major environmental trigger), but it doesn't strike me that her level of health probably changed - in other words, just a different manifestation of the same unhealth, much like a different symptom of the same disease.
At the same time, I would be very genuinely surprised if your friend had never shown any hints of CP response prior to her shift - especially from an ENTP! Do you think it's possible that you just didn't get to see it much? I know that personally, I rarely show my bursts of CP in public - rather, they're generally reserved for the few people closest to me during argument of issues I care deeply about. Yet they are there, and they also tend to fuel some longterm behavior on my part that would perhaps be surprising otherwise - but I tend to keep it more under wraps than phobic response, perhaps in part because I grew up in a very supportive and Fe-valuing household.