Q: Is it wrong to publish conference papers in journals?
A colleague was queried for publishing a paper that he had already presented an a conference in a journal. The argument was that once an article is presented at a conference, it should not be published again.
It is not clear from your question whether your colleague's study was published in the conference proceedings. While it is perfectly acceptable to publish a study that has been presented at a conference, if the work has already been published in the conference proceedings, it should not be republished in a journal. This is because academic journals want original contributions that have not been published elsewhere.
Having said that, since a conference paper is likely to be a working paper, a more detailed version could be sent to a journal for publication. Of course, the paper must be updated and expanded to suit the aims of the journal; the general convention is to have at least 30 percent new content. Also, you must change the title, cite the conference paper, and mention in the cover letter that the paper's content has been presented at a previous conference.