Your friend must be stubborn and still drank cold water and ate food not suitable for coughing. And he’ immune systm is compromised at his age.

After he recovers, tell him to go carnivore and eliminate junk from his diet. Also, always drink warm water (hotter the better) no matter how hot weather is or how thirsty you are.

Practicing carnivore and drinking warm water has kept me well and strong enough ever since I started. No sore throat, cough, sniffles or fever.