Here's my not-so-scientific hypothesis partially formed by my last relationship: women frequently get "BFFs" (other women best friends) who they're extremely close to emotionally, sometimes much more than their own boyfriend/husband. And sometimes (frequently?) these BFFs are extremely "toxic" women, who give them terrible relationship advice, try to break up their relationship because they hate the woman's bf/husband or are jealous, etc. So, as you said, after the breakup, the BFF feeds her resentment, continually telling her what a POS her bf was, and how she's better off without him even if it means being forever single, etc.