// top that on your GM/FS TOP (depending what you're using)
#define dcmd(%1,%2,%3) if (!strcmp((%3)[1], #%1, true, (%2)) && ((((%3)[(%2) + 1] == '\0') && (dcmd_%1(playerid, ""))) || (((%3)[(%2) + 1] == ' ') && (dcmd_%1(playerid, (%3)[(%2) + 2]))))) return 1

// on OnPlayerCommandText you put this: dcmd(command_name,number_of_characters,cmdtext);
public OnPlayerCommandText(playerid, cmdtext[])
{
   dcmd(heal,4,cmdtext); // the word 'heal' has 4 characters, so 4 there
   // any other stuff
   return 0;
}

After all callbacks of your GM/FS, put the command:

dcmd_heal(playerid,params[])
{
   new id; // id which will be healed
   id = strval(params); // params1 = id which will be healed
   if(!IsPlayerConnected(id)) return false; // if the id isn't connected, nothing happens
   if(id == playerid || playerid == id) return false; // if the id is the player which used the command, nothing happens
   else // Otherwise..
   {
      SetPlayerHealth(id, 100.0);
   }
   return 1;
}