This is almost not a child question but a question about any friend/person you care about. They're old enough that just setting restrictions is going to make them replace one unhealthy habit with another if the underlying issue is not fixed. Is it really better if you forcibly take away the computer and they replace it with going to pool halls all day?

If I had a friend that was wasting their life away, I'd try to find out what they were interested in doing in this decade or 5 years of their life and then I'd try to help them and motivate them to begin on it. You'd then probably have to actively try to help along the way because they probably don't have the skills yet to push through at every single minor roadblock they might hit. Basically you'll have to repeat this motivation/help hourly, then daily then weekly for a long time.

"People discount motivational speeches because it doesn't last. Well, neither does bathing and that's why we recommend it daily." -zig ziglar