Your files are protected by your password. If you create her an admin user, she can't get to your files unless you expressly give the permission. Admin user because it sounds like you want her to have control. If a standard user, you're still involved, have the control and she'll likely be asking you to enter the password to get certain things done. It all depends on who you want to actually administer the Mac..
But keeping your files on someone else's computer is not a bad thing necessarily, but why do it? If you need your files, transfer them to your computer. Her MBA is portable after all, and she and "her" Mac may be gone when you need something.
Now is the time to get more familiar with your Mac. All computer users need certain basic skills and that knowledge and know-how comes with working outside your box so to speak.