。
She doesn't know even the simplest thing about science.
Literal
She [topic-は] science [regarding-に関しては] no-matter-how simple thing [even-さえも] not-know.
Two emphatic patterns work together. ~に関しては picks out 'as for science specifically,' setting up an implicit contrast with what she might know about other topics. Then どんなに~さえも piles on emphasis: どんなに ('no matter how') normally pairs with ~ても ('even if'), but here it's been bent into adjective-modifier + さえも, giving roughly 'not even something however simple.' The どんな(に) family is one of the building blocks of Japanese emphatic patterns — どんな (kind), どんなに (degree), どこ (place), いつ (time) all compose with でも, でさえ, and ても to express universal or extreme cases.