Originally Posted by samiakuma
That one is for Ubuntu, most articles are for Ubuntu... I do not have Ubuntu, but some other form of Linux.
I need to know if this risks losing his files.
The principle is the same. I personally would never give a 100% guarantee that you won't damage his files. Before you try anything of this nature it would be STRONGLY RECOMMENDED to back up all important data.
If you could have him back up all the files and then try, I'm really not sure how much harm you could do, worst case scenario you just reformat the computer, put his linux distro on it and restore the files from a dvd or something. If this scenario is unacceptable then I wouldn't recommend dual booting. Murphy's Law.