Any computer can have multiple operating systems on it. (e.g.you could have Windows 8 and Linux if you wanted to). You need to partition the hard drive, which tricks your computer into thinking that there are two separate drives, each with their own operating system. Then you install a different operating system onto each partition. When you boot up the computer you'll be asked which one you want to use.
It's not a particularly tricky task but it can be tedious, as you need to install all of the necessary software and drivers onto
both partitions.
This Microsoft page was provided for people wanting to add Vista alongside an existing operating system but the principle is the same irrespective of which operating systems are used:
http://windows.microsoft.com/en-gb/windows-vista/install-more-than-one-operating-system-multiboot
This is also relevant:
http://www.wikihow.com/Install-Two-Operating-Systems-on-One-Computer
It's unlikely that you'll be able to buy a laptop with two versions of Windows on it (other than to special order). I'd recommend having a word with the guys at A C Computer Warehouse; they might be able to offer your friend something:
http://www.accomputerwarehouse.com/
Otherwise try a local independent dealer. (Places like PC world can't even offer you Windows 7. It's Windows 8 or nothing there!)