Soweit helfe ich dir, den rest musst du machen.
new bool:spectating[MAX_PLAYERS]; // ganz oben
//bei onplayerconnect:
spectating[playerid] = false;
//cmd
ocmd:spectate(playerid,params[])
{
new pID;
if(spectating[playerid] == false) {
if(sscanf(params,"u",pID)) return SendClientMessage(playerid,color_red,"Benutze: /spectate [ID/Name]");
SetCameraPlayerBehindPlayer(pID);
} else {
SetCameraPlayerBehindPlayer(playerid);
spectating[playerid] = false;
}
}