If you happen to meet Celia, please say hello for me.

Literal

By-chance Celia [with-に] meet-[conditional-たら], greetings [object-を] relay-[polite request-てください].

よろしく ('regards, kind regards') is a polite set greeting; よろしく伝える means 'convey one's regards to X' — a standard way to ask someone to pass on greetings to an absent party. 会う takes the met person with に.