Rarely is this even foreshadowed before it all breaks loose.It's a very fast Character Derailment which serves only to cut off the audience's investment in Alice/Charlie and drive Alice into Bob's arms.If done too abruptly and without foreshadowing, viewers will feel that the writer has used an Ass Pull or Deus ex Machina in order to get the Official Couple together.
So Dee solves the problem by diverting viewer sympathy away from Charlie.
Typically this is done by turning him into an utter Jerk Ass.
Alice's sweet suitor starts showing traits that were never there before. He reveals (or Alice discovers) that he's cheating on her and has been for some time.
When the author supports a relationship or resolves a Love Triangle by turning the hero's romantic rivals into complete jackasses. Alice loved Bob, but Bob didn't or couldn't return her love, so she left him. Charlie is sensitive, rich, smart, and gives her everything she wanted from her relationship with Bob — and . Meanwhile, Bob realizes what he had when Alice loved him and now he wants her back — if he can win her heart before she marries Charlie and all hope is lost.
There's only one problem: Alice has no reason to take him back.
She's met her perfect match, she's in love and life is bliss.
And why should she believe that Bob, after not-loving her for so long, is genuinely in love with her now? He had his chance, he blew it, and now everything is about Alice and Charlie.
Fortunately for Bob, the writer (call her Dee) is in his corner.
Bob's predicament leaves Dee with her own set of problems.