How do I copy a user profile between Macs?
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.