Sorry for the delayed response; this has been a very busy development time for me.

To enable a user to edit only a single page, first give them a WordPress role of Subscriber (or a custom WP role which contains only the read capability).

Then open up the desired page using Pages > All Pages > Edit. Scroll down the the "Editors" metabox. Expand it and click the Users tab. Key the username the filter box or click "show eligible users" box if necessary. Check the unbraced checkbox next to the username. Click Update to save the page.

Note that the braced checkbox is for assigning the role to subpages.