Thanks Brian. The problem was authentication, but not in the _netrc file, but rather in its location. We had it in the user's home directory, which is where it is for me and another user for whom it works. For this one user, it needed to be in her "Documents" directory (on windows), because for her, R is putting the libraries under \home\Documents\R\win-library rather than \home\R\win-library.

I'm not sure what causes this difference in R installation or why Rlabkey changes where it searches for _netrc depending on where it is located. Maybe it is making assumptions about where it is located by default and doing a relative path search?