So from that perspective I can say… the internet and video games were very much filling a void in my life. It took spiraling into deep depression and thoughts of suicide to realize I needed to make the change myself. I suspect any degree of forceful action wont work sufficiently.
Find out what this kid wants out of life, actually get to know them, something my parents never really did. And this might mean asking to play video games with them.
Once you know what they want, what they truly want, you can help provide the tools and guidance for achieving those things outside of a television or computer screen.
Making the effort to actually connect with the person before suggesting an alternative lifestyle is key.
That being said, this person is a young adult and many people live happy lives spending lots of time in front of a screen.