The early stages of Alzheimer's are often relatively mild and people with the disease can still make their own decisions. As the disease progresses most do eventually lose the ability to make their legal decisions as NorthJersey.com points out in "Planning becomes vital following Alzheimer's diagnosis."
The article suggests that people who receive an Alzheimer's diagnosis should get a general durable power of attorney, a health care power of attorney and a living will.
The powers of attorney will allow the person to appoint someone else to handle financial and medical affairs, respectively, when the Alzheimer's patient is no longer able to do so. The living will is used to give advanced directives about what type of treatments doctors should employ to keep the patient alive.
These are important documents to have and getting them after an Alzheimer's diagnosis is solid advice. However, even better advice is not to wait until after receiving such a diagnosis. All three documents are part of general estate planning. There is no need to wait until you know you will need them to get them.
Reference: NorthJersey.com (Dec. 27, 2015) "Planning becomes vital following Alzheimer's diagnosis."