If it would not work it would not be running on > 800 Servers, mine included.
Where is the problem?
To answer your question, use your code like so.
CreateTimer(HUD_INTERVALL, Timer_DisplayHud, GetClientSerial(attacker), TIMER_FLAG_NO_MAPCHANGE);
Then in the timercallback retrieve the client of the serial and only display the message to him.
public Action:Timer_DisplayHud(Handle:timer, any:serial)