。
She's almost never late for her commitments.
Literal
[topic-は] rarely commitment [for-に] be-late thing [subject-が] does-not-exist.
約束に遅れる ('be late for an appointment/commitment') uses 約束 in its 'made plans, promise' sense — distinct from the abstract 'promise' which English tends to focus on. ~ことがない here is the negation of ~ことがある ('there are occasions when X') — 'there are no occasions when X happens.' The full pattern めったに~ことがない is 'almost never has occasion to X' — slightly more emphatic than めったに~ない. Punctuality is culturally weighted in Japan: arriving even five minutes late to a social or business commitment is widely considered impolite, so this sentence reads as a clear positive.