As of the current version of the game, Red is basically the backup. If the player gets to the end of the showdown with Betsy without meeting Shawn on a prior day, Red shows up.
You can beat Betsy by yourself. One of the prerequisites is to evolve your pokemon but the TM choice doesn't really affect anything. I'm probably going to rewrite this a little because the only move that matters is Rock Slide and you get that move no matter what. It's kind of a fake choice, which I don't like.