。
She was kind enough to let us know.
Literal
She [topic-は] kindly [emphasis-も] us [to-に] informed-for-us.
知らせる (しらせる, 'to inform, to notify') is the causative-derived form of 知る ('to know') — literally 'to cause to know,' used as the standard verb for notifying or informing someone. The に marks the recipients of the information. The action of proactively informing someone is valued as considerate behavior in Japanese culture.