。
She is well-versed in that issue.
Literal
She [topic-は] that issue [in-に] is-detailed.
詳しい literally means 'detailed, minute' but as a predicate paired with に, it shifts to mean 'knowledgeable about, well-informed.' This に-marked-topic + adjective construction is a productive Japanese pattern: 数学に強い ('strong in math'), 歴史に詳しい ('knowledgeable about history'), 機械に弱い ('weak with machines'). Compact and idiomatic — far more natural than the literal-translation alternatives a learner might reach for first.