Been there, done that. With film, Hasselblad on tripod. And with 35 mm SLR, unsupported. With a good stance you can do it with a dSLR without tripod as well. It's like Torus says. Pick a bright day and set your cam's sensitivity at 100 ISO or whatever is its lowest setting. Best way to do it is to let your buddy drive the car a continuous 35/40 mph in a perfect circle (ideally around a roundabout). You stand in the exact middle of that circle, plant your feet firmly on the ground, and pan smoothly tracking the car while exposing at 1/30th, 1/15th and 1/10th second (shutterspeed priority; let the aperture take care of itself).
Twisting your upper body you'll only be able to track the car part of the circle. That's OK. You can 'pick him up' again when he comes around again.
Your buddy must be prepared to drive dozens and dozens of those circles. The more, the better. It may be boring as hell, but it's neccessary if he wants his blurred photos. And you must be prepared to track, pan and expose all those times. The more, the better.
Make only one careful exposure per circle. If you fire off a number of frames consecutively, all of those will be jarred, thus useless duds. So save yourself the trouble (and dissappointment).
It ain't easy to get a good exposure. But it can be done with care and patience.