Sometimes I get a paper for review, and I am not sure about certain points in the paper. It could be really helpful for me (and for the paper) to discuss these points with a friend who might understand these issues better. Suppose the friend is unrelated to the authors, so has no conflict-of-interest. Is it, in general, allowed to show the paper to the friend and ask for opinion?
The main point of peer review is that the reviewers and authors are blinded, which you would not necessarily violate in this situation. In the past, I have been given articles to review where I did not understand the statistical methods used for that particular study. I felt that it was a larger disservice to write a positive review about the article without knowing if the stats were completed correctly with adequate results than to consult a colleague about the issue. I printed out the manuscript and asked her questions about the methodology without sharing data or findings with her. I did not email or share copies of the manuscript with her. I also did not tell her what journal I was reviewing for to further blind the process. I felt that this was appropriate and you could do the same.