My father-in-law has POA over my Mother in law who has Alzheimer's. If he dies my husband has POA. What does my husband have to legally do?

BTW, no one "has POA over" someone else. This is a common misconstruction, seen often in posts here. That individual acts as a proxy for and on behalf of the individual granting the power, and has legal responsibilities and standards with which to comply. It's a fiduciary relationship, in which the individual acts on behalf of and for the benefit of the person granting the authority.

Are you asking what your husband has to do in terms of specific proxy responsibilities if/when he inherits those responsibilities? Or are you asking what's necessary legally to transfer responsibility to your husband?

If the issue is responsibilities, it would include asset management, financial transactions and obligations (bills), real estate maintenance including payment of taxes, as well as anything else that falls within the parameters of the original POA. If it was prepared by an attorney, it's more likely to be thorough than something downloaded from the Internet.

Another aspect which may warrant legal advice is to determine whether or not laws, requirements and standards have changed over the years. I get updates from a law firm with an estate planning practice on various legal changes to that field, some of which definitely require legal involvement and interpretation.

Your husband will be taking over all responsibilities now done by the husband. He and your father-in-law should now be sitting down together to discuss where everything is located, as he will have a whole lot of duties in getting himself registered as POA. Quite honestly, expensive as Lawyers can be, it is often worth an hour of time to understand what the duties are, and to decide if this is a job your husband can take on/wishes to take on. You are thinking ahead, and that is a good thing. There are many books available out there, as well. Is your husband ALSO the executor of their "Trust" or their "will"? This is the time to get everything straightened out, as your hubby is likely to be in charge of all decisions if this death occurs.

If your MIL is a Medicaid recipient, your husband will be responsible to fill out her reapplication every year.

There are POA documents for both medical and health care decisions. Sometimes there is the same POA for both and other times these duties are split with different people responsible for each. POA’s represent the best interest of a person when they can’t do so for themselves. They make decisions for medical care to be provided, or not provided, and financial matters when someone can’t do so. POA ends upon the person’s death