If she has a 3G sim card, she puts her sim in the phone. Connect to iTunes and activate. It will prompt her to add a data plan if she doesn't have one.
Done.
If she doesn't have a 3G sim, call AT&T or drop by the store.
There is no need for formal activation. Both of your lines are already iPhones with the correct data plan and Visual Voicemail.
Your can just clear your iPhone "Reset > Erase All Content and Settings" and your employee can just restore his current backup from the iPhone 3G to your 3GS. If you don't clear it your contacts and apps can get mixed up.
Once your 3GS is reset, it will show the Connect to iTunes and your employee can just connect and restore his current backup. This assumes that both phones are at the same level of OS 3.1.3 or 3.0 for example.