I have been asked informally to proofread a paper by a colleague on topic X with an application on Y (constituting about 10% of the paper). After my comments (and those of others), the article was submitted. While it was in review, I looked at Y in more detail for my own work. My own research has led me to believe the conclusions on application Y are quite incorrect.
The paper is now accepted for publication and only typographical edits are accepted, so although I communicated my concerns to my colleague it cannot be included in his paper, even if he wanted to. Would it be wrong for me to prepare a paper on Y myself, disagreeing with my colleague's article (because I had foreknowledge about that article)? If yes, how long should I wait before considering sending something in myself? Naturally, I want to stay on good terms with my colleague.