With a smile on your face replace the shrub. Then send samples of roots, trunk, and foliage to plant diagnostic lab at your expense without telling your client you are doing so.
When the resutls come back, show them to her even if they point to a misapplication (highly unlikely). Either way you have proven yourself to be a professional who cares about your clients. She will cetainly this tell to the 500 people she knows. She may even offer to pay for the shrub & lab.
You will also know more about shrub death than you did before and how to go about diagnosing other problems.
The nation that destroys its soil destroys itself.
Franklin D. Roosevelt