Distilled water

Any site I could show my friend, that will tell them distilled water will not hurt the computer sys if a water cooling system leaks?

Pure distilled water wouldn't hurt a computer system. The problem is that if it does leak it comes in contact with surfaces which have dust and other contaniments which will make the water mildly conductive. Thus, it will still be able to hurt the system... Though using it slightly betters your chances and reduces corrosion possibilities.

Pretty much any water leaking inside the system will be bad mojo.

The water by itself doesn't conduct electricity, it's the stuff *in* the water that carries it. So even if you put pure water into the system, as soon as it leaked and came into contact with any surface that wasn't perfectly clean it would start to conduct again. Search for filtered water in the forums for a couple of worthwhile threads on the subject.

Many people like distilled water because they feel it reduces the battery effect within the system and possibly has better thermal properties. It only costs a couple of dollers a gallon and it certainly can't hurt.

Unless you go with non-conductive oils in an exotic system you'll have to worry about leaks.

Hope this helps,
Mark