Your cousin should have all the info....it will give all the particulars. GM has a website that shows their employee discounts, though I don't remember off the top of my head.
Most of these programs are two tiered--one tier for your cousin (the best discount) and another for his "friends and family" or for suppliers (versus actual employees.) I think GM calls them the X plan and the S plan, but I don't remember. I bought a car under the GM supplier program when I was working for a firm that did work for GM.