I agree with all of the above, however, sometimes there is undue influence - where a person (usually a parent) acquiests to the "raiding of the account" because they are afraid of loosing the attention/help/affection of the child.
Usually thi child is added to the account, and withdraws money for their own use rather than the parent (look for an uptick in spending, or spending on themselves rather than the parent). In that case, I would recommend dilligence and seek guidance from elder abuse (or whatever the county name is) so you do not have a customer with no money and no support when the funds are drained. I had been at a bank where another sibling sued the bank as their sister wiped out close to a half million dollars from a "joint account" due to the sister being added because of "undue influence", and spending the funds on herself. They did recover the Mercedes though.
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Integrity. With it, nothing else matters. Without it, nothing else matters.