In our environment users have to login using there domain username/password, so if you log in with your iPad then log in to your computer then change your password, your account will be locked as the iPad has the old password. Too many attempts using an incorrect password will result in the account being locked. I can't tell you how many times a user will become upset with Service Desk because they forgot they logged in at home and keep getting locked out
We had this problem with Apple Cinema Displays (most recent one). When the USB keyboard was connected to the display, there was a delay of input. The users would hit enter, but their entire password didn't go in. After doing that over and over, they'd get locked out.
I plugged the keyboard directly into the Mac Pro. All was good from there on out.
@EliasG The posts below are good posts; I would add, from a more general level, what variables exist with this user that does not exist with other users in your environment? This particular user is doing something different or has different devices; else, other users would be locking up as well. If you know what time she locks up, you can cross-reference the time with logs, either in the client (/var//log/system.log) or server side, such as AD.