In a PVEH, there is no way to determine what player broadcasted the variable. Instead, if this information is desired, the mission developer has to include it in the broadcasted variable. This has negative security implications, as clients cannot be trusted and a client could claim to be anyone they wish.
Some kind of player identifier should be provided to the event handler. This could be a player object or perhaps the client ID.
An array is already passed to the PVEH containing the variable name and value, so it should be trivial to add another element to that array containing this.