There were actually more reasons why my friend prefers the older MLP. The villains thing was just one of them, and was the only one I could remember at this time.
With villains, I do agree that villains aren't necessary, but they can make a story more interesting and add more drama. FiM doesn't have villains (unless you count Nightmare Moon as one, and maybe Discord), but it does have antagonists (such as Gilda, Diamond Tiara and Silver Spoon) which help create a conflict and interesting story. And some of the episodes didn't have obvious antagonists, like the episode with Zecora. In that episode, the antagonist(s) were, in my opinion, Twilight's friends, as they were the ones who kept pressuring Twilight into believing Zecora was a witch until she finally gave in.
Antagonists don't need to be evil or destructive. They just need to create conflict in some shape or form, and they can be one of the good guys, one of the opposition, themselves, nature, etc.
