I'm not sure what you're asking. The grammatical structure used in the sentence John might say is called reported speech.
"Rosy said she didn't like you."
As for the expression that could be used to describe John's action, it depends on the nature of the conversation between John and Rosy. If Rosy trusted John and thought she could openly talk to him about Pete, but John turned out to be nothing but a snitch, you could say John ratted her out.
I think it's perfectly appropriate given it's a language forum and these abbreviations are fairly common in textbooks and dictionaries when presenting and explaining structures where syntax is key. As long as you don't use them in normal, everyday conversation, or worse, formal register, and only use them when discussing language, like here on this forum, I think you can use them.