americanlawn is right. Obviously a root issue. Probably poor install. Not root scorned properly or installed too deep and crown not getting enough air. Do not just replace as suggested by prior post. You should be able to explain the cultural issues which caused the death. It's cut and dry, very obvious. If you just replace it you could be lassoed into replacing everything that's not your fault. No disrespect but if you can not explain this simple issue you need higher education in your profession. You also should have noticed the decline as it was occurring. It was not overnight and probably started on the outer branch tips. I'll even bet with little effort you can pull it right out of it's spot. Do this in front of the customer to prove the root issue. It will be obvious. Properly rooted plants can't be pulled out by hand in most situations. It may even break off at the crown. All this will show cultural issues. ( Not your fault ) unless you installed it.