Also, excessive pad toe-in, poorly cut housing, and poorly seated ferrules can lead to mush on top of poor centering.

If the cables wiggle more than they do on a bike with solid feeling brakes, they're robbing the motion of the cable that clamps the brakes together by wiggling. Try cutting the ends square[r] and reseat the ferrules if you can.

Personally, I like a semi mushy lever, I've always felt it helped me modulate the brakes better.

Canti levers on linear brakes are VERY mushy. Some levers have a setting for both canti and linear. Do the brake arms seem to move a normal amount in relation to the cable pull? You seem to have a sense of how brakes should feel, compare her brakes to a bike you approve of. Take note of what's different beyond just the feel.