|And then Twilight gets to set a literal boundary.|
At least, it's closer to the color of her magic than Cadance's.
On television, we have "Three's a Crowd" by Megan McCarthy and Ed Valentine, which much like "Castle Mane-ia" before it is what might be called a stealth arc episode. On initial viewing in sequence, it has little to do with the season's arc, but in hindsight it sets up two important elements in the story: Fluttershy's task to monitor the migration of the Breezies, which will lead directly to acquiring her key, and Twilight Sparkle's evident dislike of Discord but apparent unwillingness to outright reject him as a friend, which sets up her segment of the quest for the keys in the season finale.
The bulk of the episode is spent on that latter point, and so once again the show misses an opportunity to assert that friendship is not transitive--that Discord's friendship with Fluttershy does not give him any claim on Fluttershy's friends. Instead, the episode appears for much of its length to be treading into Friendship Is Mandatory territory, with Twilight Sparkle appearing to accept Discord's claim that they are friends even though she has never been shown to say that Discord is her friend or to be friendly toward him.
I have discussed before why the notion that everyone can and should be friends with everyone else is toxic, and will not rehash that argument here; instead, I will note it is particularly disturbing when Discord is involved, because as in "Keep Calm and Flutter On" he is openly abusive toward his "friend." His abusive behavior begins almost from the start of the episode, when he first brazenly lies by claiming that Applejack and Rarity agreed to nurse him, then manipulates them into reluctantly agreeing after all. This is a variant of the abusive technique known as "gaslighting," in which the abuser persuades the victim of something they know isn't true, undermining the victim's confidence in their own judgment and increasing their dependence on the abuser.
This action also puts them in a position to be infected by the (nonexistent, but that's unlikely to stop a being of Discord's power) blue flu, which together with the departures of Rainbow Dash, Pinkie Pie, and Fluttershy earlier in the episode render Twilight almost completely isolated, allowing Discord to pursue what he implies through denial at the end of the episode is his main project, isolating Twilight from her friends, particularly Cadance. There is no evidence of any particular scheme past that, so his likely motivation is simply jealousy and possessiveness--again, common traits of abusers.
He then spends the bulk of the episode forcing Twilight and Cadance to serve him with the goal of preventing them from enjoying their time together. He makes a series of absurd demands with the justification that they are small requests necessitated by his illness--again, trying to undermine his target's sense of reality in order to substitute his own framing--culminating in the canard about the giant worm-guarded flower.
In the end, Discord claims this was all a test of Twilight's friendship, which is again a typical abuser thing to do. He reframes his malicious and cruel behavior as an achievement on Twilight's part, a way in which she can redeem herself for being a poor friend (when, of course, she is not his friend at all). He even tries to claim that he expected Twilight to enjoy being tested because she likes tests. This is both an attempt to excuse himself from blame for the events of the episode, and an attempt to establish dominance by positioning himself as someone who has the right to test Twilight the way, say, her mentor Celestia does.
So Discord is again thoroughly vile, and again the series refuses to acknowledge that Twilight has the option to choose not to be his friend in response. But nonetheless it is able to somewhat redeem itself by doing exactly what "Keep Calm and Flutter On" didn't: hold Discord accountable for his actions. Twilight is rightly furious when she learns Discord was faking being sick, and tells him so. More importantly, his efforts not only fail, but backfire: Cadance has a great time fighting the worm with Twilight, and then Discord becomes actually sick as a consequence. As Rarity says at the episode's end, he got what he deserved. Fluttershy is of course much kinder to him, but it is not enitrely clear that Fluttershy has a concept of "deserves."
Although the episode does not quite go far enough, it does at least ultimately acknowledge Twilight's right to set boundaries by having her tell off Discord and leave him to the care of someone who might actually want to help him. It further highlights this as a positive action by punishing Discord with real illness, acknowledging that he deserves it, and rewarding Twilight by having her day with Cadance go well after all.
This episode also serves to clarify Discord's behavior in "Princess Twilight Sparkle," placing both episodes in the context of "Keep Calm and Flutter On." Specifically, in both episodes Discord is still just as malicious as he has been since his introduction, but proceeds more subtly than in "The Return of Harmony." In both episodes he provides a cover story that makes his cruelty deniable, positioning himself as a trickster mentor rather than the trickster villain he actually is. In "Princess Twilight Sparkle" his cover story is that he is nudging Twilight toward understanding that she needs to work to maintain the same relationship with her friends as she had before her ascension; here it's that he is insecure and needs to prove Twilight cares about him. In both cases, however, his goal is simply to sow chaos in Twilight's life, most likely as revenge for the events of "The Return of Harmony."
It's becoming very clear that Discord needs a friendship lesson of his own, and of course Twilight providing him one--modeling for him how to be a good friend, making him (at least partially and temporarily) recognize how utterly self-centered and terrible of a friend he is--will be the crux of the season finale. It's unfortunate that, in setting this up, the show created the impression that Twilight is in any way obligated to teach him to become a better friend; it is dangerously close to the "I can fix him!" attitude that can so easily increase one's vulnerability to toxic and abusive people.
Next week: Some singer the target audience has never heard of. Heck, most of the bronies were born after his heyday. No, this one's clearly for the parents.