There are multiple ways you can achieve this -
Email - you can email the page or section. Save it locally (say on desktop)
Open it in OneNote on the other computer (it will open in Open sections)
make changes to it. close the opened section. Email back the updated
section. Open it in your computer, copy it back from the updated section.
(in this case you will still have your original section which you emailed)
USB - Yes, in this case you have the additional advantage of getting the
entire notebook copied on your USB stick. (This is the folder under
Documents \OneNote Notebooks\) open it on the other comp, make changes and
bring back the updated notebook. (Again, you would still have original NB..
you can close it and open the new one with updated changes)
If both the computers are on same network, then the most optimum solution
would be to use folder sharing to share the folder and give access to your
friend to make changes to your notebook.
hope this helps a bit.
Let me know if you have any specific questions.
