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She sends a letter to her pen friend in London twice a month.
Literal
She [topic-は] month [per-に] 2-times London [possessive-の] pen-friend [target-に] letter [object-を] sends.
The per-unit に in 月に2回 ('twice a month') marks 月 as the unit and 2回 as the count — same particle slot you'd find in 週に三回 ('three times a week') or 一日に二時間 ('two hours a day'). Choice of counter matters: 回 is the everyday choice for events and instances, while 度 has a slightly more literary flavor. 手紙を出す ('to mail / send a letter') is the standard action collocation. ペンフレンド is a borrowed compound that sounds slightly dated today, where 文通相手 (the all-Japanese term) or ペンパル might be more current. The pen-pal practice itself peaked in mid-twentieth-century language exchanges and has been quietly replaced by email and online language partners.