This a bit tricky but possible. Firstly I would put the project and any events it needs in a separate library, because you will have to send the complete library to your colleague and even this library will probably exceed the 5 GB limit of Apple MailDrop or other free file transfer services (and it will take a very long time to upload). Possibly easier is to post the library on a memory stick. If you have included any media from other Apple libraries (iTunes, Photos etc.) you will need to consolidate the library (see: http://help.apple.com/imovie/mac/10.1/#/mov882dee351) so that these items are copied to the library and not referenced to the originals. Because Apple has frequently changed the iMovie library database format between updates (even minor ones), your colleague must be using exactly the same version. Unless the memory stick is Apple-formatted (not the usual default FAT - formatted) and fast (USB 3), they should copy the copy the library with the finder to their hard drive then copy it back to the stick after editing.
Note that if your colleague is going to provide a separate part of the movie then it would be simpler for them to create it as a separate project then combine the two projects later.
Geoff.