Good call on the cider vinegar, it helps move mucus in the case of chest infections.
If the bird sounds chesty, snotty, coughing, sneezing etc, then a trip to the vet for some antibiotics is in order.
Baytril or Tylan are the ones you want.
Your neighbour wants to treat the rest of his flock as well, as once one bird is diagnosed, guaranteed another will start the same symptoms.
They can do this by asking for soluble meds & adding it into their drinking water