Is this possible? Prevent Live-Linux from booting and grabbing your files without encrypting your disc? I am a medium-experienced Power User with knowledge about most of the stuff (studying this since two years) and I know something about how Linux and Windows work; I have screwed up countless installations and stuff like that (dont want to bore you, just want to tell, that I know what I am talking about).

To the point:
A friend of mine has a MacBook, running OS X Lion and a hard drive with about 320 GB (not encrypted, at least not the part that is important) and he tells me that forcing a change of his password would automatically activate a self-destruction (complete wipe of the drive). This could be true and would be no prevention, since I don't want to change the password, I just want to get the files from his drive. So I can simply boot Linux from CD/USB and mount his drive, copy everything unencrypted, and done.

That's what I told him, his answer: "Well, that could work, but my Mac will automatically detect this and wipe the drive before anything gets copied." The main question: Is this bullshit?

From what i would expect, this would work like:
BIOS --> Linux --> Mount Disc --> Copy --> Start OS X --> Detect and wipe

So files should be there without the wipe, but he said it won't work, and OS X would be booted before Linux? I was really surprised, so I want to ask you guys.

He told me that he will give me his MacBook and I can try to get his files, but I don't want his disc to be wiped, so I ask here: is it safe to try it out?
Your friend has no idea what he's talking about. You don't even need to use a Linux disc - you can use single user mode or a Mac OS X install disc to copy things or make changes to the filesystem.

To prevent someone from booting into single user mode or from an unauthorized disc or external drive, your friend can set a firmware password. However, this, too, is easily bypassed if you can open the computer and remove the drive, or alter the memory configuration. Simply put, if someone has physical access to a computer, encryption is the only way to protect your data.