Need help with event flag.

You won't need to edit the ROM itself -- what he did was edit the game's memory with a cheat code, replacing the value at memory address 7E/1F49. (A bit -- or an event flag -- would have been set during one of the events you've finished that stopped those NPCs from showing up anymore.) By reverting that flag, it means the NPCs can show up again. Unfortunately, he didn't actually say what he changed the memory address TO, so I can't help you there. Changing the memory byte back to #$AC afterwards restores the events you've completed so you can continue the game as you were.

he used a game genie! I had to watch that part of the video again.
well I suppose that I cannot do that, so I guess that I will have to continue without the esper 

Edit: I DID IT, the only thing that I had to do was change the value of the adress 7E1F49 to 55, thank you!