、。
She ignored him, but it ended up being a thoughtless thing to do.
Literal
She [topic-は] him [object-を] ignored [but-が], that [topic-は] resulting-from [topic-には] discretion [subject-の] not-existing act was.
結果的には ('as a result, in hindsight') frames the second clause as a retrospective evaluation — the action looked one way at the time but turned out otherwise. 分別 (read ふんべつ here, not the trash-sorting reading ぶんべつ) means 'discretion, good judgment,' and the noun-modifying clause 分別のない ('lacking-discretion') uses の to mark the subject of がない inside the modifier — の and が are interchangeable as subject markers in such embedded clauses, though の has a slightly more formal feel. The clause-final が is the soft 'but' connector, distinct from subject-marking が.