Rear calipers on the front?
That is most likely the issue. In most cases 80% of braking is done with the front brakes, rear brakes/calipers (wheel cylinders too) are generally smaller. Given that disc brakes are more efficient than drums, disc brake calipers are more exaggerated.
Changing the master cylinder to a smaller bore and the pedal ratio to provide more leverage against the system may help the situation. Without a proper knowledge of the parts being combined, your bud is just guessing.
Brakes and steering are the two most important elements in a car, don't screw around.