you can save your project in mxv file contining all ressources, after you compress and send using werransfer or any sharing website like Dopbox to your friend.
. . . . what's the best way to send a project to a friend so they can work on it . . . .
If you mean send him the MVP file and all the source files in the project there are 2 options:
File, Backup copy, Copy project and media to folder.
This will copy everything needed for the project and some special files needed to load the project. Then put the entire folder, you backed up the project to, into a Zip file and send this to your friend via any sharing site eg Dropbox, Google Cloud, Microsoft OneDrive etc where you can upload the zip file to - beware some free sharing sites have file size limitations and you do not want everyone to be able to see the shared file.
In MEP 2018 there is also the Share, Send with Send Anywhere, Send project and media.... option - follow through the dialogues saving the files in a folder of there own.