、、。
She knew him better than anyone else did.
Literal
She [topic-は] him [object-を], other [genitive-の] everyone [subject-が] knew [than-よりも], better knew.
A comparative with an embedded clause as the standard. 他のみんなが知っていた ('everyone else knew') is a full subordinate clause with its own subject (が) and tensed verb, then よりも attaches to compare against it. The main clause ~よく知っていた ('knew well') describes the topic's level. Comma-set-off relative comparisons like this are common in narrative writing — they give space for the comparison clause without making the sentence feel cluttered.