Does the receiver have tone controls for treble and bass? Many do, though sometimes the options are hidden in the electronic setup menus. Post the manufacturer and model number of the receiver and I bet someone will know, or will be able to download the manual to find out.
Adding an equalizer to an existing receiver is not possible within the bounds of practicality and good sense (in most cases -- again, post the model for more specific details). It is usually cheaper and works better just to replace the receiver.
I never use the "sound fields" (theater, live, etc.) in my receiver because I think they sound bad (unnatural and weird). I suspect most people on here avoid them like I do.
It is an "equalizer" or tone controls that can adjust treble and bass. An "amplifier" takes the tiny signal from your CD player (or other "source") and "amplifies" it to power the speakers.
Treating the room (rug on a wood floor, heavy curtains over windows, tapestries on walls, or using audio-specific panels and bass traps) can also help to tame "hot" treble and take some "one-note-boominess" out of the bass. If your friend has a specific problem with how his room sounds, describe the room and the sound problem and you might get some good basic treatment advice.