As long as we're on the topic of heavyweight fabrics, I saw one quilt many years back that was made with different colors of cotton twill from old cut up chinos. The thing was heavy as all get out, but my friend kept it in the back of his pick-up for spur of the moment hiking trips. Wish I had a picture. It was raw edge & frayed, but so cool. I'm not sure what it was quilted with, if anything, but I'd guess likely an old wool blanket from the Army/Navy Surplus Store. I seem to remember it had a flannel back.
Personally, for the best chance of surviving several hours on the side of the road (I have AAA & it's happened but fortunately I just had a flat tire so the heat still worked), I'd want a lightweight cotton quilt plus 1-2 solar emergency blankets. Quilts are cozy, but have no moisture barrier. In addition, while heavier quilts can start out warmer, as they trap perspiration against the body, he will get chilled.
Also, I know it's a HUGE no-no, but maybe consider some sort of vinyl/plastic sack for the quilt unless he's going to store it inside of some waterproof case because otherwise the quilt will get wet in the rain & be useless for keeping him warm.