Your friend would need to change the entire axle assemblies to get 8 lug hubs. Then, the hubs may not be the same pattern or diameter, so his Chevy wheels may not fit. Additionally, trailer axles use wheels with 0 degree wheel offset. Most automobiles/trucks use wheels with an offset, usually in the 16 degree range.... This would mean you'll probably have some clearance problems trying to install a truck wheel in the wheelwell of the trailer, even if the lug pattern is the same.
It probably would be very expensive to even attempt a modification like this and in the end, the truck wheel would likely not clear the trailer wheelwell.
I'd say it's best to leave it alone and find a place for the trailer spare. Possibly something like the "BAL Hide-A-Spare"...
http://norcoind.com/bal/products/con...-a-spare.shtml