This is, of course, owing to her charming tendency to not understand the odd concepts called 'etiquette' or 'reality' when doing so gets in the way of her feeling right. A normal person would know to not take sides in a break-up (especially when it's forced) because she doesn't have all the facts at hand. St Elli, Guardian Spirit Of Being A Blundering Ass doesn't see that as being a good thing because she thinks every messed-up relationship is her own screwed-up dalliance with a succession of boys who either saw her as a clingy and pathetic joke or, due to her volatile personality and her veteran dad with a low tolerance for hippies, as a threat to their person. Since being rejected by someone is the only way she's ever been on the other side of a break-up, she assumes that this boy has cruelly dumped her.
She also assumes something manifestly not the case when she tells herself that she's doing a good thing telling her to flush this boy because she's certain that she would have wanted someone like her to come along and tell her the same thing back then. This sort of nonsense is why there is the phrase 'not even wrong.' We don't know much about her past other than maundering about tear-stained pillows when the longed-for ones wound up in the clutches of the dangerous body language squadron but we do know that she probably still resents her mother for making the same sort of dismissive cheering-up she's pulling now and we know that no girl in Molly's position is going to go "Gee! You're right! I should simply go on with my life because he isn't the be-all and end-all of existence! I apologize to everyone for having inconvenient feelings and will now join the pep squad!" because that's not how they roll. To assume otherwise because it would make life easier is to deliberately misunderstand a fundamental fact and be not even wrong.