Legally, he stands in the shoes of the customer. If the customer could make the claim, he can make the claim. In turn, if you have a defense you could assert against the customer (the passage of time since the transactions occurred) you can assert it against him as well. (Reg E's commentary contains a "wrinkle" on this point.)
Practically, he may not be in a position to assert that the customer did not conduct the transactions unless the customer has literally been confined to the nursing home during this period.
Reading between the lines, this sounds like an elder exploitation situation. That does not automatically make your bank the underwriter of the illegal activity.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.