This is advanced technique and in the end, Can make for a bigger stronger bushier more bud sites plant. Your friend is correct, but it will take much longer for this plant to grow to where you want it, than if you didn't chop.
What most don't take into account, is that when doing this, you will then have a root scructure and size that is now feeding a half the size plant, root to shoot ratio goes way up, and plants become extremely robust and buds get huge.
imagine a plant, that has roots in a pot feeding the upper growth, now you chop that plant down, and the same roots are now feeding a plant half the size, you get monster growth, stability from a larger root to shoot ratio plant above that holds sugars and nutrients in it's larger root scructure for smaller plant above.. stronger plant with huge roots compared to top above that feeds that upper growth that is now smaller, and roots dont stop growing as long as pot size is not restrictive and this root to shoot ratio stays with the plant for life and you get an overall better plant in the end.
This is advanced technique.
Just some blahblah for ya...