She was forced to provide it in order to get a new joint account passbook, or to cancel the joint account. But she, as a joint "either/or" account holder, could continue to drain the joint account legally
"...she, as a joint "either/or" account holder, could continue to drain the joint account legally".
Not without advising the bank of the death of the joint account holder, she couldn't. Deaths have to be advised to institutions such as banks, that's why they asked for it now!