Let me preface this with, only one user is having this issue. Everyone else can log-in to the Joomla administration just fine.
Originally when I found that the single user was receiving a 403 when landing on the administrator page, I checked to make sure all directories were writable as per the directories listed under Site Information. I've also confirmed that the administrator directory has 755 permissions, and that the user has cleared her cache.
Also, the site was recently moved from one server to another. Changed the A record on the old server to the IP of the new server.
Thoughts as to why a single user would receive a 403 error only in the Joomla administration page?
After chat discussion, it turns out that the server has been moved recently and since then the problem started.
The problem seems to be cached dns on the user's side, so the user has to clear dns cache, or wait for the provider to clear dns cache