。
She gave in to his marriage proposal.
Literal
She [topic-は] his [genitive-の] proposal [to-に] lost.
求婚に負ける ('lose to a marriage proposal') uses 負ける ('lose') in its 'give in, succumb' sense — the same verb used for losing a competition. Implied subtext: she resisted at first, but eventually his proposal wore her down. The に marks what she gave in to. A more romantic, slightly literary phrasing than just 受け入れる ('accept').