Karl naturally won't relapse, he's not really the same Karl in the first place, after all, but Erika might. Karl will need to keep in touch and maybe come up with new methods to make her stick to it. Of course simply the fact another noble of her age, who is in a similar position to hers, is there for her will go a long way to help her. Nothing he says is empty platitudes since he's just as miserable himself. He's also there not to share the misery but to make things better. This should work. I hope whoever it was in the last panel won't try to hurt the progress.
Maybe Tiana wouldn't mind if he patted her head? Who knows.