Requesting a copy of the PhD thesis from an author?
A PhD thesis is usually a published work, and is normally archived in a university's library. Almost all dissertations are available via reprints. So you could ask your own library how to order a copy of the thesis.

Alternatively, as you suggest, you can contact the author directly. As Anonymous Mathematician suggests in his comment, you can ask for a copy of the thesis, but you should definitely explain why you're interested in the thesis. However, it may be possible—and even more so for a humanities thesis—that the author is currently preparing it for publication, and may therefore be reluctant to share it via electronic means. However, it is just as likely that they're willing to share it.