Eviolite is an item which increases the defense and special defense of a Pokemon that is not fully evolved by 50%
So lets say you had a Magneton with 100 Defense and 100 Special Defense. That means with an eviolite, it would have 150 Defense and 150 Special Defense. So it doesn't boost the Pokemon's stat when leveling, just immediately adds 50% to whatever the Pokemon's stat would be without the eviolite.
Since it works on any Pokemon that isn't fully evolved, of course it would work on Magneton since it has an evolution.