How do I copy a user profile between Macs? I have an Mac Pro running Lion, my wife has a Macbook Air also running Lion.

We have the same apps installed.

I want to steal my wife's Macbook while I'm out of town on business.

To do so, I'll have to bribe her, but I'll also need to give her a seamless experience using the Mac Pro for the week.

How can I easily copy her profile, including all settings, mail, and documents, over to my Mac Pro so she can use it, and then send it all back to her Macbook when I get back?

I know there's a migration wizard, but neither of these is a new machine, and I only want to copy one profile (hers).
Run migration assistant on both Macs. If you can update each to the latest patch for your version of OS, that often is good but remember you have to always move data to a same or newer OS this manner. Moving data from a newer system to an older one is not supported.

https://support.apple.com/en-us/HT204350

It will let you bring over one user account with ease. Her password and desktop and settings and preferences will all be preserved to the maximum extent possible. If both macs are the same version of the OS - it's even very easy to delete (back up) the older user and move the newer one back when the portable returns from its trip.