。
She had no choice but to turn down his proposal.
Literal
She [topic-は] his [genitive-の] proposal [object-を] decline [other-than-ほか] there-was-not.
~ほかなかった ('had no choice but to,' literally 'there was no other than X') is the past form of ~ほかない. Built from ほか ('other, else') + ない ('not exist') — 'there was nothing else.' The verb that fills the slot is in dictionary form (断る). A useful set construction for expressing reluctant necessity. プロポーズ is the loanword counterpart of 求婚, common in casual modern Japanese.