If she happens to be late, please pass this message along.
Literal
By-chance she [subject-が] late-came-[conditional-たら], this message [object-を] relay-[polite request-て下さい].
遅れてくる is a compound verb 遅れる + 来る meaning 'to come late.' 伝言する ('to relay/leave a message') is a formal compound built from 伝える ('to convey') + 言 ('speech'). The polite request form てください is marked with 下さい in kanji for slightly more formal tone.