USAFRet :
Whether you have a valid OS license or not, obtaining a Windows install via torrent is simply a bad idea.
Windows 7:
If you have a valid Retail OS license, go here:
http://www.microsoft.com/en-us/software-recovery
Windows 8.1, go here:
http://windows.microsoft.com/en-us/windows-8/create-reset-refresh-media
If he doesn't have the disc, he probably doesn't have the license. But yes, Windows torrents can be bad if you're not careful. Make sure to check reviews and comments and only use trusted sites that don't do things like censor comments or have shady uploaders. Sites like torrentz.eu and Pirate Bay are good examples of sites to NOT go to.
If it is an OEM license, contact Dell.
If he doesn't have the disc, he most likely doesn't have the license and can't access it from within Windows since he can't boot.
But yes vibhujoseph, as this guy said, it's usually a bad idea to download from torrents, but this is only true if you're not careful and you're being stupid and pirating things like games and stuff.
Be sure to check reviews and don't go on shady sites like torrentz.eu and Pirate Bay which can cause all sorts of problems from the site alone.