Call your state's equivalent of the Department of Human Services or Department of Financial Institutions and inquire about the correct role for your institution. Verify what you are told before you spill the beans (some public advocates are a bit overzealous in their explanation of what the bank's responsibilities are).
In general, banks are either "voluntary" or "mandatory" reporters of financial abuse of the elderly; i.e. your law may allow you to do it or it may require you to do it.
Again, get a citation to statute or regulation and read it or have the bank's attorney read it. Do not just take advice from an employee of a public agency.
As an aside, only the customer can assert a forgery or alteration. You may begin reviewing her items and, if appropriate, use "irregular signature" or "apparent alteration" as your reason for return if you do it before the midnight deadline. That will heat things up for sure.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.