He needs a media box like a Dune Smart H1 or a Popcorn Hour box. Then he also may want the NAS if he wants 4 bays. The media boxes usually have a bay for 1 HDD, plus USB and eSATA connectivity for adding more drives. They also can act as NAS which allows direct access from a networked PC.
However, if he's working with HD video/audio, he should be aware that network speeds make transferring such large files very time consuming. Any drive attached to the PC can be shared, allowing direct access to files from the media box, so the NAS is not strictly required.
I prefer multi-bay HDD docking stations, then just swap in the HDD that has what I want. 2 and 4 bay docks are reasonably priced. Such a dock can be attached to the networked PC or to the media box, or you can have one in each location and swap HDDs back and forth.
If I were him I'd go with a Dune plus a 4-drive bay and leave the bay attached to the PC and shared.