Watching tv a lot can have ill effects on health, mainly eyesight. It also decreases our ability to imagine and our creativity. Most importantly it is a waste os precious time.
You can tell him what all he/she is losing out on and give them some time to work on something which make one productive like:
~ practising skills
~ enhancing knowledge by reading
~ exercise, yoga ,etc
~ indulge into hobbies
~ play outdoor games
~ read books, newspapers, magazines, etc.
Hopefully this has answered your question!