There are two things that create noise in mechanical keyboards. The type of switch, and in less expensive and poorly constructed keyboards, the key bottoming out on the plate on the bottom.
Good gaming keyboards are not cheap. You could upgrade him to one with Cherry Red switches, which would reduce noise, but not eliminate it completely. As far as silent switches go, the only keyboard with "silent" switches on the market is the
Corsair Strafe found here, which you can get with either Cherry MX Red, or Cherry MX Silent switches.
Other options for noise reduction that are considerably less expensive are purchasing a key puller and o-ring kit,
like this one. You simply pull the keys, put the o-ring on the switch, and...