I only brought up the friendship beam because your post brought up the fact that Cozy ended up getting turned to stone as if that somehow makes her irredeemable despite no attempts being made to redeem her. And the fact that Frenemies showed potential for them to be better makes it all the worse that they were never given any help in realizing that potential. When did Discord show that potential even before he was turned to stone the second time? The only reason he started showing it in the first place because he was given the chance and help to do so, something which the trio and by extention Cozy, never got.
At least Discord had Fluttershy, who still trusted and tried to help him despite knowing what he was like, what he did and just how dangerous he could be. No one knew that Cozy was secretly evil and as such, were not trying to reform her and when she was revealed they gave up on her almost immedietly. A school, even one based on friendship, is not a rehabilitation program. What Discord got is. And again, Discord's actions in s8 and 9 were after he got his second chance making them all the more worse. If he deserved that many second chances, then its only fair that at least one attempt at rehabilitation should have been made with Cozy.
And yes, Cozy would try to manipulate others, but we don't know that she can't be changed because no attempt was ever made to do so. If Discord was treated the same way Cozy was, he would never have reformed and would never have tried reforming. Cozy deserves the sympathy she gets because of the way she was treated as opposed to people who were at a time just as bad as her.