Ian's suggested procedure is the way to do it. Along those same lines, you can create a new iMovie library, copy the project into it, and then transfer the library that contains the project to a thumb drive or other external drive that is formatted Mac OS Extended (Journaled). Then plug the thumb drive into your wife's computer where she can open the library and edit the project. .
For this to work both Macs must be running the exact identical version of iMovie 10.
An Mp4 actually would work to a limited extent. You could share out your movie to an Mp4 file and then transfer that file to your wife's Mac. She could then import it into a newly created iMovie project and edit it to a limited extent. She couldn't change any embedded edits that you made, but the Mp4 could be split, trimmed, segments deleted, new clips added, titles added, effects added.
-- Rich