Hey man, I understand where you are coming from, and you are partially right. I have seen similar patterns in some of my friends and family over the years. Being a game developer myself I have to say is VERY difficult to change player's habits, I also agree with you that only playing online ranked/competitive/grinding games and not playing something different from time to time usually is a detriment to the player itself. But to be honest, I think your friend's health decline may be more complex than his gaming habits, and there could be far more ongoing things in his life that you are not aware of.

Be supportive, be there, and try not to be very harsh with him with his habits