What prevents you from verifying the keyboard in the Keyboard Viewer? That can tell you a lot!
For example, if the Alt/Option key is not depressed in the Keyboard Viewer, yet you get results that show the Alt key is being used, then there's some other software issue - such as a utility that provides settings that allow the Alt/Option key to be used through software.
Wired keyboard, or wireless?
Go into the hard drive, then the Library folder. That would be the main Library, and not the user's library.
Open the Preferences folder in that main Library, and trash a few files:
com.apple.BezelServices.plist (Lion or newer will also have a second file by that name, with .lockfile added. Trash that one, too
Also, trash com.apple.keyboardtype.plist (there's probably a second file there, too)
After trashing those files, those are system-wide preferences, so restart that Mac.
If that does not fix the problem, then you need to test with a third keyboard, which you should test that the problem does not exist on a different Mac - or, alternatively, test the user's keyboard on another Mac.
What prevents you from verifying the keyboard in the Keyboard Viewer? That can tell you a lot!
For example, if the Alt/Option key is not depressed in the Keyboard Viewer, yet you get results that show the Alt key is being used, then there's some other software issue - such as a utility that provides settings that allow the Alt/Option key to be used through software.
I'm thinking that an out-of-control keyboard would leave the Mac almost unusable, and be really annoying to the client.
Or - is the client using a non-Apple keyboard, where the command and Alt keys are reversed by default? That keyboard would have a Windows key, eh?