It doesnt work so easy, besides record, he will need to capture images by cl_avidemo, but before that he will need to set com_maxfps for example to 25 for shity movie, then find some jpeg2avi converter like Quake Video Maker or any other, so he could merge all jpegs he get from cl_avidemo into single video and etc.
But he can use much easier way, record through xfire or fraps, if he dont need hq movie.