。
She's a former bank employee.
Literal
[topic-は] former bank-employee [explanatory-です].
もと as a prefix attached to a profession or role means 'former, ex-' — もと先生 ('former teacher'), もと総理 ('ex-prime-minister'). It's productive across many roles. 銀行員 ('bank employee') is a compound: 銀行 ('bank') + 員 ('member, staff'); the suffix 員 is one of the workhorse occupational suffixes in Japanese (会社員 'company employee,' 公務員 'civil servant,' 駅員 'station staff'). The polite です completes the simple identification.