Cho didn’t betray the DA

Marietta was not a character in the films. The films portrayed Umbridge finding out about the DA because she gave Cho veritaserum.

I kinda feel sorry for Marietta, she was put between a rock and a hard place. Cho pressured her to join the DA with her, so that Cho wouldn't be alone when she was trying to flirt with Harry. Then, she must have felt horribly conflicted because her mother was a Ministry official, and had ordered her to not do anything that might upset Umbridge. She paid dearly, and perhaps unfairly, for her decision to side with Umbridge and the Ministry. The 'SNEAK' pimples never quite went away.--EJ