Since you have no idea who made the rifle, then keep the loads simple. 1-66 twist is a roundball shooter. So have your friend start with 70 grains of FFg and a .015 patch with a .490 roundball. Then have them shoot groups and see how tight a pattern they can make. The rifle should be safe up to 100 grains of FFg powder.

Look over the rifle close and see if there is any manufacture marks. Then you can contact that maker and ask them for a manual. In the mean time keep the loads 100 grains or smaller.