Troubleshooting noises, grinds and the like can be a bitch. Could be a driveshaft flange, differential bearing, rear wheel bearing etc..
Does he have a GSL or GSL-SE model? Both models came equipped with LSD rear-ends.. I would hunch that if the noise just started after a hard launch- check the driveline flange (u-joint) first, then the wheel bearings, then the differential. Here's a nice guide too:
http://www.mazdatrix.com/g2.htm