I suggest you go with your friend to the local district office. They have a standard form for a last will and testament and will help you in filling it out in a way that is foolproof.
At the very most, a bank may need a court decision regarding the appointment of an executor of the testament before paying out money to the heirs designated in the testament.
Banks want to safeguard against the possibility of statutory heirs (parents, siblings, descendants, etc) making claims after having paid already to the beneficiaries stated in the testament. Legally, it is the obligation of the executor of the testament, not of the banks, to research the existence of possible statutory heirs. Any person wanting to make a claim would have to sue the executor, not the banks, but at some banks the management may not know this.