Although grammatically correct, it has a different meaning than what is intended. "...fala em..." implies that she is considering doing it or having it done (by a private detective, maybe) whereas "...fala sobre..." means she has done it already, and is commenting on the whole situation and the events that followed.
It's more or less the same as "sobre", although you may hear it used in place of "em", but very rarely. "Falar em" is very colloquial, also. You'll never hear anyone say "o livro fala em..." because that's for people only (yeah, I know books don't talk, but we use "falar" for books in Portuguese all the same). On the other hand, you can say "o livro fala sobre..." or "o livro fala de..."
Sorry to be harping on this subject after being talked about so exhaustively.
I just need to ascertain the accuracy of the following:
Can FALAR be alternatively followed by EM, DE or SOBRE?
- "Ela fala de seguir o marido."
- "Ela fala em seguir o marido."
- "Ela fala sobre seguir o marido."
postalblue writes that "falar em" is colloquial, but I have read in "ciberdúvidas" that it is quite acceptable in EuP: "falar em fazer algo."
Here's a list of prepositions that follow "falar".