With the zone 5/6 designation and that white wall with a south facing aspect you could probably grow something for zone 7.* Here's some that come to mind.
Acer saccharum 'Barrett Cole' aka Apollo maple.
This tulip poplar grows about 15' wide with nice fall color.
Here's a list of trees for limited spaces.* The crab apples will give your client the flowers and the apples, if they are the small apples they will feed the birds better.* The Red Cascade mountain ash is also a great possibility.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.