。
She pretended not to notice.
Literal
She [topic-は] feigned-ignorance [object-を] did.
知らん顔 is a fixed compound literally meaning 'unknowing face' — to feign ignorance, deliberately pretend not to recognize, or pointedly ignore. The 知らん inside is a fossilized contraction of the negative 知らない (from 知る), preserved here even in standard speech because the whole phrase is a set noun. The construction [noun]+をする 'do/perform an action-noun' is at work, the same pattern behind くしゃみをする ('sneeze') or 顔をする ('make a face').