Agreed... Here's my suggested procedure, which is a little different from what Rounik suggested (with apologies)...
Save channel strips for each of your friend's tracks as Rounik suggested. I take it that your friend played acoustic instruments? If so, then he's provided you with audio tracks and in that case, while you're in his project, select all of the audio [b]regions[/b] in the arrange page and CMD-C copy. Now those audio tracks are in the clipboard.
Close that project out completely without saving.
Open YOUR version of the project and create the requisite number of channel strips for your friend's parts. While you do this, do not use CMD-X or CMD-C, as it will blow out the clipboard.
Once you're done setting up the channels, use the command "paste at original position", not CMD-V.
Voila, those audio regions will now be in your version of the song.