。
She took it for granted and had him pay the bill.
Literal
She [topic-は] naturally [possessive-の] thing [as-として] him [to-に] bill [object-を] made-pay.
Two notable structures here. 当然のこととして ('as a matter of course / taking it for granted') uses ~として to frame her attitude. 勘定を払わせた uses the causative: 払う → 払わせる ('make/have someone pay'). The combination paints a picture of someone who unquestioningly expects others to pay — the causative implies she directed or allowed the action, and 当然のこととして shows she felt no hesitation about it.