Low thymes are flowering at the moment and look beautiful, with the added bonus of attracting bees etc.
London pride is a saxifrage and there are plenty of different types of saxifrage which flower through spring, starting with S.oppositifolia (purple) and carrying on with named varieties such as Boston Spa (yellow), pixie (deep pink) and some bright whites.
There are two types of cushion phlox - douglasii which is very small but perfectly formed (my favourite is Eva), then there is the larger variety which is probably the one your client has bought and is more impactful, making cushions up to about 3' across in time.
Campanulas have various forms including cushion types which are a beautiful blue and flower for quite a long time. I have several varieties which have self seeded around the rocky walls of the waterfall in my pond - beautiful
There is a tiny plant called Fairy Foxglove (I've totally forgotten its proper name) which will self seed on the sides of a wall as well as the top - it doesn't have a long season but as part of a succession it's wonderful and so delicate, if it likes the situation (my last one has died!)