For me, the very frustrating thing about Cho's portrayal in the movie was that in the book she wasn't the tattletale! :)

http://www.hp-lexicon.org/wizards/a-z/e.html#Edgecombe_Marie...

Marietta Edgecombe is Cho's friend, randomly introduced for the purpose of ratting out the Army.

Unfaithfulness to the book aside, I agree with your analysis of the way they used Cho's character. Not cool of them.