I'm not sure how do you "trigger a message", so tested with Admin Console.
FMPro become foreground, but the message dialog has 30 seconds life time. After the time, dialog disappear and FMPro return to background.
So user can easily miss the message.
Client A could create a record in a message table. The message would be entered into a text field. Client B would have to use something like an OnTimer script to periodically check for that new record and bring it up when detected.