In the book she didn't. It was Marietta Edgecombe. In the movie however it was Cho. At one point Harry tells Cho that he heard Umbridge gave her a hard way to go. Later, when Harry is caught by Umbridge and the Inquisitorial Squad, Umbridge calls Snape into her office. She demands that he bring her more Veritaserum (a truth serum). Snape advises her that she used the last of the serum on Miss Chang.
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To make Cho tell her where the Dumbledore's Army meetings were held and who was involved, Umbridge forced Cho to drink Veritaserum, which forces the consumer to tell the truth. (But only in the films, which have nothing to do with the books)
There Is no evidence from the book to tell us, but it may have been Cho Chang.
Cho Cho Cheng was born on December 29, 1985.
This was basically to save introducing and building up a new character in the movie. There was a lot of storyline to fit into the movie, so they saved time by changing the snitch to Cho, much to the demise of many book fans.
Margaret Cho's birth name is Margaret Moran Cho.