In case he didn't start mapping yet, tell him to:
- click file > create a new difficulty
- insert new difficulty name and adjust other things, click ok
- click yes when asked to remove all objects
You need to get the .osu file, or at least the content. So if he mapped his gd, he should give you the .osu file from song folder (the one which is mapped by him of course) and you'll put this file into your song folder. Update your map after this and that's it.
Post in help forums next time for such questions