。
She turned down his invitation.
Literal
She [topic-は] his [genitive-の] invitation [object-を] declined.
断る is the everyday 'refuse, decline' verb — covering invitations, requests, offers. Less formal than 拒否する, less active-resistance than 拒む. The plain past 断った is neutral, just stating the fact of the refusal. Note the same kanji 招待 appears in noun form here ('an invitation,' the object of the refusal) and as the verb 招待する ('to invite') — same pair, different role.