Claudia,
Ajuga usually does better in part sun, so there must not be very much sun under the trees your friend wants to plant in.
Purple wintercreeper will grow anywhere, and is nice because it turns purple in the fall.
I have some sedum planted where it only gets afternoon sun for a couple hours of the day and it is doing fine.
I like sedum, you can buy any kind and try it out.
They don't grow real fast though, they will spread afer a couple years, but she won't get instant gratification.
Mabey she could try a little bit of different kinds of sedum to see which one grows best.
I love sedum and they look so nice when they flower.
There is always climbing gardenia too, radicans, but that would probably be pricey if it is a large area.
Then there is ajuga, she could try that and see if it takes, you never know.

Thanks, butterfly! I'm favoring the ajuga, I think... she does want fast results and with our climate here it will have until well into November to get established. But I always turn to the folks on this forum when faced with a question I'm unsure about. There may be enough sun for ajuga... I have it in spots that are nearly all shade. I appreciate your input!

if she really likes sedum there are a few that do spread quite fast. S. tectorinum (chinese paddle sedum) can handle shade and goes to town. Because the leaves look like tiny ping pong paddles it has a different appearance than a lot of sedums. Also, s. acre spreads pretty doggone fast and will do fine in shade as well as sun. Key things with sedums is decent drainage, esp in the shade. They like reasonable soil- you don't need to put them in sand or gravel, though they will grow in those, but they are faster if they are started in soil. But clay isn't very hospitable for them because it's too dense and they can rot out. If it were me, i'd try 3 or 4 things and see what wins the battle. Sometimes things surprise you.

This sounds like a catch-22 situation to me. If the sandy soil in deep shade under a canopy of trees is not supporting any weeds then very little will thrive there without some changes to the light levels or improving the soil. But if you improve the soil and increase the light you open the site up to all sorts of weeds that a non-gardener won't want to deal with.

There's reason the ground is often bare under certain trees.

LOL @John! :-D My friend may just be out of luck! The only thing they've ever tried to grow there is grass... no luck, but they do have some weeds they keep trimmed. I've suggested she rake in some top soil before trying the ajuga, and that she doesn't invest in too many plants before giving a few a try. (But I have to wonder about folks who want a nice yard but don't want to work to get it, even if I DO love them! That's why they make landscapers, I guess...)

At my last house I had lots and lots of trees and lots and lots of shade. I planted ferns and hostas and they all died (some took longer than others). After years of working the area I finally noticed that there were few weeds growing in that spot. If it won't support poison ivy then very little will do well in that kinda of dry shade. Trees suck up all the nutrients and water within their drip lines. Tree roots will invade any improved soil you bring in and suck it dry also. One trick I used in particularly bad spots was to sink a plastic flower pot into the soil (kinda hard to do with all the tree roots) and plant a fast growing tropical foliage houseplant in it. Things like Swedish Ivy and Wandering Jew will cascade across the ground and cover an area about 5 feet across in a few months. When I wanted to water I just pointed my hose at the planted pot. It wasn't a permanent solution, you have to replant every spring, but it kept the area under the oaks from being completely dead.