Well, asking mac vs. pc is flame bait. I'll try to give you some objective advice. I have a mix of Windows based PC's and mac's in my own home.
If all of the software your friend needs is available for mac, then that strikes out the biggest advantage pc's have over macs.
With that said, a pc will be cheaper and it will be more upgradeable. You could build a killer windows based pc for €1230.
An iMac will have a small desktop footprint, and apple quality is very high. They come with very nice displays and have that "mac-factor" that some people find attractive. For windows based pc's, you need to do your research to sort the good from the bad. You can have a pc that is a complete pile of crap, but you can also have a pc that exceeds the quality of a mac. Building your own assumes you can spot the good from the bad when it comes to hardware.
In summary, if your friend is looking for a sleek all-in-one computer, all software is available, and money is not an issue then an iMac will suit him just fine. But for high-end power, flexibility, upgradeablilty, and bang-for-buck you can't top a Windows based custom built PC.
If your friend wants an iMac, then he'll probably get an iMac and there's nothing wrong with that.
It's funny, because macs are actually pc's. It's really Windows vs. OSX that is the debate.