。
She's deeply impressed by John's bravery.
Literal
She [topic-は] John [genitive-の] bravery [at-に] is-admiring.
感服 (kanpuku) is a heavier word than the everyday 感心 (kanshin) — both translate as 'admiration,' but 感服 carries a stronger sense of being thoroughly won over, of acknowledging that the other has exceeded one's expectations. Built from 感じ ('feel') + 服する ('submit, yield'), it literally describes yielding to feeling. The に marks the trigger of the admiration. The continuous ~ている here describes the state of being in admiration, not an ongoing action — Japanese routinely uses ている for emotional and mental states that English would express with simple present (knows, loves, admires).