I was asked to referee a paper. The topic is within my field but not in my exact area of research. The paper seems all good and fine, but I struggle to judge the originality or whether they miss relevant work. I have a friend/colleague in my department who works in the same area. Would there be any problem with asking for their input on refereeing the paper?
I would assume that it is a fairly common practice as long as no conflicts of interest arise. But, to be sure, this is really a question to address to the editor. You might have to say a few words about the colleague.
Some situations, of course, require confidentiality. Most do not as long as the normal rules and expectations apply.
I know more than one journal which (at the stage of review submission) explicitly asks reviewers whether they prepared the review alone or discussed the manuscript with other researchers (and if yes, ask for their name too). So this is totally fine (except obvious situations when you ask help from one of the co-authors of the manuscript, of course), just don't forget to mention it to the editor.
Because even when a paper is technically in your field, you might not necessarily be an expert in the specific topic, there is always the possibility to reject an invitation to review a paper. If you did not accept the invitation yet, I would honestly rather suggest you politely decline the request, stating your abovementioned reasons. If you have already accepted, than I would inform the editor(s) that your colleague (without any conflict of interest) is joining in the review.