I'm not a Mac user, but from what I've seen on other forums (e.g. Outlinesoftware.com), the main Mac programs which resemble UR are Devonthink, Yojimbo, and perhaps Together (formerly Keep It Together).
You might also have a look at the comparative review here:
http://www.viewfromthedock.com/2008/...review-part-i/
Another possibility is the Evernote beta, which supports Mac.
I don't know whether any of these programs have scheduling capacity, as UR does, in case that's important to your student.