。
She let him know she had arrived.
Literal
She [topic-は] him [to-に] arrival [object-を] informed.
知らせる ('to inform, let know') is the causative-like derivation of 知る ('know') — 'to make known, to bring to (someone's) knowledge.' Recipient takes に, the information takes を. Common in many contexts: phone calls, emails, announcements. 到着 ('arrival') is a Sino-Japanese noun; the action of arrival is 到着する.