Hello
Your friend has not recovered from jet lag. This is related to the fact that he has not fully adapted to the climate, environment, diet and other aspects. He will get better after a while. It is recommended to do more things for yourself during the day. Although it will be tiring, it will help you sleep at night. Adjust your mentality, keep a happy mood, and don't be too depressed. Also eat foods that will help you sleep. This is a problem that many friends who go abroad will encounter.
I hope my answer can help you