They just need a letter that says they will wire the retairment check of your mother mothly, as she is foreigner. Make sure they use an oficial page from de agency and stamp the letter. Every foreigner that try o open a bank account here they make an investigation and usually takes arround 1 month the process. The letter will just make it easy for you and for them to open the account because is part of the internal procedures for the bank when is a foreigner that is opening a bank account.
only for non registered foreigners.
Foreigners who have their Residency,
just show up with their cedula and open as many accounts as they wish,
in Dollars, Euros, Pesos, checking and/or savings accounts etc.
my partner opened his bank accounts with Banco Popular with his passport, before he received his residency/cedula, which was in process at that time.
they asked for the letter from the homebank,
the homebank(in Austria) knew what letter that is(i cant tell you how it's named or so, but the banks know what form to fill in for a foreign bank on such case), and Banco Popular was fine with that and cleared the accounts to be used from there on.
i can not comment on US-american specific rules, as i have no knowledge about their banking rules/requirements for their citizens abroad. by reading thru different topics lately, it sounds that you have to fill in every year a form, which is something like your Tax-Declaration(sorry, i don't know how it's exactly named/spelled), which you have to fill in/proceed even that your income is well below the Tax-Free-Margin of yearly earnings.
Mike