。
She rejected his offer of help.
Literal
She [topic-は] his [genitive-の] aid [genitive-の] offer [object-を] rejected.
拒絶する is one of the strongest 'refuse' verbs — 拒 ('refuse') + 絶 ('cut off, sever') = 'cut off and refuse,' implying an emphatic, often absolute rejection. Compare 断る (everyday refusal), 拒否する (formal/legal refusal), 拒む (active resistance). 援助 ('aid, assistance') is formal Sino-Japanese vocabulary, common in humanitarian, financial, or academic contexts.