You can only do this if you didn't sign up for the current Unlimited Disk and Transfer plan. So if you're on one of the regular plans, read on...
I'd create an FTP/Shell user for her, then set up her domain as Fully Hosted, as owned by that user you just created. You can give her limited access to the panel here without giving her access to your billing, sites, etc.
Otherwise, she'll just have access to only her website directory, plus any email accounts you set up for her. If she needs a database, you can set that up, too, with its own user.