It sounds like the brakes work fine, but the rider needs to modulate them properly to stop safely. Disc brakes are great because they work well even when they are wet, and they concentrate the braking force neat the axle. If you slam on the brakes in a moving car (without ABS), you lose control and slide, so you modulate the brakes. The same applies to bikes.
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Paul Pinigis
Owner of Origami Bicycle Company