Any potting soil that drains well will suffice, but caution your neighbor that any hardy plant grown in a container (especially smaller sizes) is much more at risk for winter root damage as temperatures fluctuate. Plus, containers dry out faster than in-ground plants, so they must be checked regularly during mild spells for watering needs. The type of potting soil is up to the gardener; many have peat moss or coconut coir as their base material, though some are converting to a higher percentage of "forest products" (a more nebulous term that might include wood fiber, bark, and/or related tree-derived components).

Miri