My friend had a fever, can he drink cold water to cool him down? Does fever has any relate with viruses?

Fever. Viral infections typically induce fever. Cold water can help hydration but a fever is the body's attempt to kill the invader. The use of Acetaminophen is used for fever but maybe a doctor should be consulted.
Yes. Water will help with hydration which is important when someone has very. Very often a fever can be caused by a virus. When febrile a pain reducer such has tylenol (acetaminophen) or Motrin will help with relief of symptoms.