His friend, is violating the Mac OS X License Agreement, specifically the restrictions to use one copy of the Apple Software, (b) to use it on a single Apple-labeled computer, (c) not to lend or redistribute the Apple software.
Since his friend failed to comply with the License Agreement, his rights under the License terminate automatically so essentially he is running an unlicensed copy of Mac OS X now too.
Nagging about piracy may be questionable, but condoning piracy is unacceptable.
There are instances where a EULA is not viable under certain circumstances. I will concede that the software is not mine and as a licensee I should only do what the EULA states, but in reality no 'real' crime is being committed, it is more of a moral issue than a criminal one.
There have been similar arguments with the Vista EULA and virtualization, I am not trying to compare the two issues but give an example of legitimate uses of software where the EULA doesn't make things easy. Personally, if I purchased a license I would like to use the software for legitimate purposes on the same machine, or maybe another machine. But again this is thin ice we are treading on because this argument goes both ways and I acknowledge that.
In the end there are just some rules that while important on a large scale, aren't as important under certain circumstances and I think testing for a trial period is one of those. Again this is only my opinion.