Abrogail presents a rather more complicated case to the Elements of Harmony than most people.

Harmonizing an Equestrian villain is usually a matter of removing the influence of dark magic, as with Nightmare Moon, or turning them to stone until the stars align for their redemption, as with Discord. Against a truly irredeemable being they would—well, you'll see. But Abrogail is none of these things. She is, right now, Good—but only under the influence of drastic nonconsensual mind alteration, which is definitely not harmonious. Neither is making someone Evil. 

In the end, however, the reality is that while Discord flipped her alignment, the things he showed her were the truth, and the Elements won't make her forget them. Her alignment aura changes back to Lawful Evil. Her desire to do something about Hell doesn't change. Even if she were perfectly selfish—which she isn't, when she thinks about it—she's one of the people going there.

"Are you okay? What happened to you?"

"No, but there's nothing anypony can do about it. And I'd really rather not talk about it." She tried to say 'I'm not allowed to talk about it', which is more the truth, but it turns out Mephistopheles won't let her.

Twilight knows enough about Cheliax, now, to have guessed that Abrogail really hasn't ever been okay, but this is the first time she's heard her admit it. She's really not sure if that's a good sign or a bad one.

"Okay. Well, if you change your mind, that's what friends are for," she says.

No, friendship is not actually going to solve this one.

"I need to talk to the Princess," she says.

"We were just headed back to the castle."

She sets off up the garden path after the other ponies. "What happened to Discord?"

"We turned him back to stone. He claimed he'd be back in a season and a half, whatever that means."

"Four and a half months?"

"Nopony who was trying to be understood would say it that way, but—"

"Yeah. Did Celestia but him back on public display?"

"Hold on. He was where?"

"In the statue garden, where anyone could release him with one sixth-circle spell—the other statues are people too. I checked."

That's dumb and also kind of fucked up. "I'm sure the Princess had her reasons," she says.

And then they're at the doors of the throne room. The faint sound of shouting is audible from within, but it's hard to make out any words.

One of the guards knocks loudly on the doors. The shouting dies instantly, and the doors open slightly. "The Bearers of the Elements of Harmony!" the guard announces.

Said ponies, plus Abrogail, file into the room.

It's kind of obvious who was shouting at the Princess, since she's the only other one in the room.

This was entirely predictable. She's a bit disappointed she didn't get to watch.

"Well done, my little ponies," says Celestia. "Discord won't trouble Equestria again, thanks to you. I hope you've learned some valuable lessons about friendship along the way."

"Thank you, Princess," says Twilight, bowing. "I've learned that if you keep dangerous villains petrified in a public garden, somepony might unpetrify them in the middle of the Grand Galloping Gala."

Celestia looks uneasy. "Well, yes," she says. "I suppose that's true. But that isn't really a lesson about friendship, is it?"

"Oh, I also learned that your friends will help you stay true to who you are, even when aforementioned villains mind control you into not that. But knowing that in advance would only have solved this slightly faster; keeping Discord's statue in a shielded vault would have prevented it altogether."

"Twilight, that may be true, but there are things you don't—"

"You know," interrupts Abrogail, "I've been to a place where people decorate their gardens with their petrified enemies. It's called Hell."