Has she checked the ph of the soil? Pieris & Skimmias prefer the acidic side of neutral. So if she didnt use ericaceous compost in the pot that wont have helped.
Did she continue relgiously watering? even in wet spells, as new plantings in such a situation will have needed extra.
Suitable groundcover plants for beneath the canopy & around the trunk- Ivies (sorry), Lamiums, Vinca, Ajuga, small spring bulbs will grow happily though these until the canopy closes over later in the yr. Cyclamen. Bergenias & Heucheras & some hardy Geraniums will cope too. More plants/shrubs will do better at the edge of the canopy.
Nerines would prefer a really sunny spot. Although the aspect is good, it may still be a wee bit shaded for them. Could try them in a pot first though?
The soil beneath the tree will be very compacted, so any plantings need to go into a 'pocket' of leafmould/compost/soil improver to give them the best start & water.
If all else fails, then a good deep layer of say bark chippings or even slate like chippings will provide a ground covering nearest the trunk.
It depends on how 'tidy' a grdener she is/wants. My neighbour has a layer of coloured slate chippings beneath a small Acer. Not my taste, but very good to look at & minimal maintenance. J.