You might try pureed baby food. Choose a meat one, and mix with a jar of the sweet potato one. You can put that in a dosage syringe, and feed. Hold the head tilting it a little upward, squeeze a little food in, then relax the head, so the chicken can swallow. Repeat several times, and multiple times throughout the day. When facing the chicken, hold the dosage syringe to the left side of the chicken's mouth. Reverse that to the right side, if you are not facing the chicken. Do NOT get any down the center hole, as it is the air passage.
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I hope that your hen survives.