All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Renting a locker in a commercial bank to keep precious things or the like is allowed since the bank does not benefit from the deposited items. Therefore, this deal is not considered as cooperation in Ribaa. The Prophet dealt with Jews in lawful matters even though they were the most involved in Ribaa as Allaah mentioned about them (what means): {And [for] their taking of Ribaa while they had been forbidden from it}[Quran 4:161].
However, it is better to deal with an Islamic bank even if there are male staff, since your mother can avoid all the prohibitions by wearing Hijab (the Islamic dress) whenever she goes there. If anyone tries to harass her, she can easily raise a complaint against him to the responsible person.