My friend borrowed my I-pod, and put all his musik on it for me to listen to.
Now, he did that of course by using the sync button with his I-tunes library, which of course wiped my musik of the I-pod and replaced it with his.
Now, as I understand it, I think to get both his musik and my musik on my I-pod at same time, I need to sumhow transfer the musik from the I-pod onto my itunes, and then sync one both his and mine musik is on the itunes library. I tried using the transfer purchases from I-pod button, but nothing popped up and nothing changed in my library and as far as I know, nothing happened.

When you open up itunes and plug in your iPod you need to uncheck the automatic sync box and switch it to manual. Then you can add all your music back on from your computer and then since you have all your friends music on it already and switched it over to manual mode the music will add and not just wipe it out. Next time you have anyone else add music from their itunes just make sure they don't have it on automatic or else the same thing will happen all over again. I think this should solve your promblem. Hope all goes well!;)

Thank you :)

But now you will have to manage YOUR I-tunes manually FOREVER to prevent the songs downloaded by your friend from being wiped out. There is NO WAY that you are going to get he songs from your I-pod to your I-tunes.
Better option is in your I-tunes check the "enable i-pod for disk use". Now allow anybody to connect the I-pod to their I-tunes. Now importantly transfer the songs from his computer to the I-pod by clicking "my computer". The I-pod will be visible as a removable disk. Does not matter whether his itunes wipes out your songs on the iPod.
Now reconnect your I-pod to YOUR PC. Your I-tunes will wipe out the playlists downloaded by your friend and load the playlists of your I-tunes. BUT THEN you click "my computer" , locate the I-pod and YOU WILL find the songs downloaded by you friend there. These you can copy to your itunes and sync.
So essentially you use the I-pod as a removable drive to transfer songs from one PC to the other irrespective of what goes on in the itunes.

Cool :) thanks