。
She didn't know what to say to him.
Literal
She [topic-は] him [to-に] what [object-を] say-if good [whether-のか] didn't-understand.
A condensed embedded question. ~たらよい (~たら + よい, 'it would be good if') is one form of the 'what should I do' frame. Combined with ~のか it embeds a wh-question as the object of 分かる: '(she) didn't understand what (it would be good if she) said.' The か here is the embedded question marker (parallel to 'whether/what'). The whole structure compresses 'didn't know what to say' into a tight Japanese pattern.