。
She knows he can't be relied on.
Literal
She [topic-は] he [subject-が] [unreliable-あてにならない] [quotative-と] is-knowing.
あてにならない is a set expression: 'cannot be relied on.' Literally 'doesn't become an expectation' — あて is 'expectation, reliance.' The whole judgement is the content of 知っている, marked by quotative と. ~ている here describes a knowing-state ('knows,' not 'is in the act of learning').