It is especially important with ALS that someone be given Power of Attorney that will allow them to sign for the patient in financial situations such as signing checks, paying bills. It can include or exclude bigger financial things like investments or running a business. A Health Care Power of Attorney gives health care decision making power and the right to all your medical information to someone if you become mentally incapacitated or unable to communicate your wishes in any way. It only goes into effect when either of those happen. Any type of Power of Attorney can be made complete or limited as to whet the designated person (Agent) controls.
Laurie has noted that you seem to be confusing Power of Attorney with other forms of Advance directives such as a Living Will. A Living Will should be copied and given to every doctor he sees as well as to the hospital he would go to. Another copy should be available at home. A Living Will is respected in most situations but is not a legal document. It also has the shortcoming of being written ahead, sometimes years ahead of when it is needed. It may not reflect any changes in the persons thoughts about what they want done, newer treatments and equipment, changes in finances, changes in the health or availability of caregivers, and definitely cannot cover all the things that could happen to us. We tend to consider only ALS progression, but would you have the same wishes if you had something fixable or not necessarily fatal happen? Would you refuse surgery, CPR, or a temporary feeding tube or a couple of days on a vent to recover completely from something?
Health Care (or Medical) Power of Attorney is an alternative but not without problems too. Health Care Power of Attorney is a legally binding document which is held up in court if it comes to that. It can only be used when you are unable to communicate your preferences. You would have to be unconscious or completely unable to blink or otherwise communicate. The moment you can communicate again the HCPOA cannot be used. It has the benefit of allowing your designated person to make decisions about your care in real time. Your specific condition, new treatments, etc. can be considered. The big downside is that you have to choose a designated person you can trust to carry out your wishes even if it means letting you die. To make certain of your wishes, it is important that your designated person be frequently in touch with you, aware of your current condition, prognosis, wishes. etc. In cases where there is family disagreement it provides the legal right to speak for you and make decisions for you that some of the family may not agree with. With a Living Will the family can influence the doctors because they can sue. Even without threatening that, the doctors will think twice if anyone is strongly opposed to following the Living Will. With HCPOA legal protection for the doctors and designated person is far more solid. The really important thing is for the person to have made his most current wishes clear to the family, and that it is his decision who speaks for him and makes the final decisions if he can't.