I am using OS X 10.6.2 so this might be different for your version of the OS.
In the System Preferences -> Accounts pane, for each account, I have a checkbox that allows/disallows the user to administer the computer. Is that perhaps left unchecked?
Is she even able to unlock the accounts preferences using her password?
My roommate has a Mac G5 running OS X.2. She has two accounts on it, one of which is the administrator. Lately she can not install any new software nor can she upgrade the software she has, even her internet browsers. When she tries, she gets a pop up telling her that an administrator password is required to install the software. Her password is correct or she wouldn't be able to log in to the system at all. I used Disk Utilities to repair the disk permissions thinking that would help, but it didn't. I even went to the extent of downgrading my own administrator level to normal and created a whole new account as an administrator thinking I could fool the system by having new permissions created. Nope. That didn't work either. So, I'm at a loss. She thinks we may have to wipe the hard drive and reinstall the software, but I'm hoping not. Please help.