I wonder if she'll learn of Cartman's true colors.
Iron Maiden, Judas Priest, and My Little Pony: Friendship Is Magic fan

Like the Simpsons and the many other cartoon shows a lot of what was previously done is essentially "erased" each episode.
Sure they'll make references now and again, but those are quite often 4th wall jokes.
Let's face it, you're talking about a show where in the first 6 seasons one of the main characters was killed off in nearly every episode. And then later it was ret-conned to be part of the story and yet no one notices, however Stan and Kyle both clearly refer to it not really being an issue in the earlier seasons. (Cartman even referred to it in the Amusement Park episode, where he got sued over Kenny's death).

But he did once own an amusement park.
My Chimp DNA seems to have lost its password temporarily. Sluggr-2

Correct. I mean between the racism, the dressing up like Hitler, his constant referring to Kyle as "Jew" and also taking advantage of transgender bathrooms.

I think the OP is referring to all of the stuff Cartman did prior to this season.

Heidi knows that Cartman is a victim, like her, in this case. She probably feels bad that initially she was blaming Cartman; only to find out that he was innocent.