。
She knows she can no longer rely on him.
Literal
[topic-は] no-longer him [object-を] reliance-[as-に] cannot-do thing [object-を] knows.
頼りにする ('rely on, count on') treats the noun 頼り ('reliance, support') as the role assigned via に — 'make him into one's reliance.' The negative potential 出来ない ('cannot do') turns the whole expression into 'cannot rely on him.' 知っている is ~ている on a punctual verb 'come to know,' giving the resulting state 'is in a state of knowing' = 'knows.' Japanese has no plain present-tense verb for 'to know' — 知る ('come to know') always pairs with ている when describing current knowledge.