Board Thread:Questions and Answers/@comment-30088034-20170108151825/@comment-27701762-20170108224935

The answer depends on what you want, but the short answer will probably be "yes." Though there's a way around it.

A True Pacifist route is not ruined by what you did prior to a reset. You could almost complete a Genocide run, and then reset and complete a True Pacifist route with no issue. All that changes is that dialogue gets altered, which gets altered in general because those are flags related to having performed certain actions or completed certain tasks. The different dialogue is essentially part of the way the game works canonically.

However, your brother is not going to face Photoshop Flowey, as once your brother defeats Asgore Flowey will refuse to transform, knowing that he'll just lose.

You can get around this issue by manually resetting the game. Locate the "Undertale" folder on your computer, which will have a few different files in it. Delete the "undertale.ini" and "file#" files. Upon restarting Undertale, it will be as though the game has never been completed before (unless, of course, you've completed a Genocide route before, since that establishes a permanent flag that is harder to get rid of). So that will allow your brother to start off a fresh run. He'll have to recomplete anything he's already completed, but he'll get the full experience.