Can a referee request a paper referenced in the reviewed paper?
Yes, if the paper is crucial to the work, I don't think there is anything wrong with asking the editor to provide it to you. I think the editor would probably check for it himself first (in order to save time), and if not possible would relay your query to the authors. As an extreme measure, if the editor was unwilling to do this, I would simply consider writing back saying you are unable to fully review the paper because access to this crucial reference was not provided.

So, in short: do not let your lack of subscriptions get in the way of doing of thorough review.



Now, there are some other ways around this. First, maybe you can find that paper in the usual ways: through interlibrary loan or on the “grey market” (ask a friend who's got more comprehensive subscriptions that yours). Second, maybe the editor offers reviewers some service that can be of help already (for example, Elsevier journals offer a 30-day free access to the Scopus database to their reviewers).