It never even occurred to her that he might be punished.
Literal
She [topic-は] he [subject-が] is-punished [quote+emphatic-とは] thought-not-even.
A passive in the embedded clause: 罰せられる is the passive of 罰する ('to punish'). The とは combines quotative と with topic/emphatic は — 'as for the very idea that ~.' 考えもつかない literally 'thought also doesn't reach' — a fixed expression for 'doesn't even occur to one,' with も meaning 'even.'