Can a MacBook Pro (Retina, 13-inch, Mid 2014) have their hard drives upgraded My friend has a MacBook Pro (Retina, 13-inch, Mid 2014) -- she wants to install a magnetic 2.5" HDD. Can I install any such drive or do they have to be apple compatible hardware.
Not possible. There's not enough room for a spinning hard drive to be connected inside any retina portable MacBook. 

That being said - hose drive would work well externally via thunderbolt or USB and internal non-OEM replacements are available. 

To install special SSDs. other than Apple branded look for resellers and online like OWC or RamJet.