Can be tricky, you will probably have to change the UEFI secure boot options to legacy mode in the bios. Boot from your Windows 7 disk or USB and remove all the disk partitions and perform a clean install. Once installed finding missing drivers can also be a pain as DELL may not list them for Windows 7. Search Dell forums for clues is my advice. Good luck.
If his PC is x86, he can install Windows 7 perfectly with or without UEFI's secure boot.
Downgrading tricks are not recommended nowadays. I higly suggest a clean install.
I took a quick look at that model and it seems that you DO have drivers for Windows 7 (only 64 bits):http://www.dell.com/...spiron-15r-5521
Having said, Im inclinded to see that most of the hardware manufactures for the hardware in your laptop will have a 32 bit version as well.