I planted limelights on the north side of my house---no direct sun but filtered bright light (against a white house) and they bloomed so beautifully last year. Should definitely be ok facing SW!

I'm in a completely different zone than you but I have a limelight in the same conditions and it does well.

I have paniculatas in a few different light settings ('Limelight' is in full sun) and they all flower for sure (full sun ones do best). As a general rule of thumb for essentially all plants, as the degree of shade increases, so too will the width of the plant, with a corresponding decrease in plant height so factor that into your mother's plans. Macrophyllas are far better for shade but 'Limelight' is a beauty no matter where she is planted.