。
She writes to her parents once a week.
Literal
She [topic-は] week [per-に] once parents [to-に] letter [object-を] sends.
手紙を出す (to send a letter) is a natural collocation — 出す here means 'to send out' or 'to mail.' 両親に marks the recipients. The habit of writing physical letters (手紙) to parents speaks to a particular era or personality, as most communication has shifted to digital formats. The plain present 出す describes this as an ongoing, habitual action.