I think you may be underestimating the seriousness of the situation. It might help to google some info on sour crop. If your hen is sick enough that she is lethargic, and it is sour crop to the point that her crop is a balloon, then food/fluid is not emptying from her crop to her stomach and she is effectively not getting any fluids or nutrition through her system. The issue is less that it could be contagious (if it is, all the other hens already have been exposed to it) and more that you hen is probably in a life-threatening crisis.