Prepared, advised, primed, given a heads-up, briefed in advance, warned beforehand, apprised, acquainted, notified, made aware, forewarned, forearmed, educated, instructed, coached, guided, mentored, tutored, trained, familiarized, oriented, introduced to, initiated, indoctrinated, clued-in, in-the-know, briefed ahead of time, briefed beforehand. All of these words can be used as synonyms for the word "prebriefed". Each of them convey the same message as prebriefed, which is to prepare someone or make them aware of a situation in advance. The choice of synonym depends on the context and the tone of the communication.