I'm giving some authoring advice to a newer writer. She has a curious convention where she uses an em dash at the end of a sentence in the narrative when she wants to indicate an abrupt change of subject (call this "case 1"). Her writing is private, hence the following is an example I've contrived:
"I think that's great," said Billy.
Frank nodded his approval. Jill was of a mind to object.—
The bookshelf behind the three children collapsed with a mighty crash.
A second convention (case 2) is that she uses an em dash in dialogue, at the end of a sentence (and often in place of a period), to indicate that a character is interrupted or otherwise intended to say more. E.g.,
"They should really build better bookshelves," Jill said.
"Yeah," Billy agreed. "And better beds. And better cabinets. And better cars—"
"And better everything," Frank cut him off.
"No. Not better everything! —"
"Whatever. It's not important."
Note that in case 2, I'm not referring to the widespread convention of using an em dash as a terminator for an incomplete sentence. Case 2 does not comprise, e.g.:
"Do you think we should clean up the—"
But Jill froze as she saw the expression on Billy's face.
None of the writing guides I've consulted on the use of em dashes suggest that either case 1 or case 2 are valid uses. However, I was also surprised to find that most don't include the "terminate an interrupted or incomplete sentence" case either, and I've seen it used in hundreds of books.
My inclination is to tell this author that her conventions are flat-out wrong and that she ought to remove the em dashes, but I wanted to check with the online community first.
Am I just out-of-date? Is either of these uses of an em dash something that can be found in English-language fiction today? Are there any examples that are publicly accessible online?