This is a known issue with Mk1 players. It's covered in the FAQ.

Messing with various grounding schemes might help. Maybe a Radio Shack Ground Loop Isolator will work?

From your description, it sounds like he's going to be running it into a mixer of some sort. Doug316 here on the BBS has experienced this exact problem (noise when running the player into a mixer). His solution was to get rid of his Mk1 by selling it to me.