Hi again,
 
Many problem posts from me, but dusting off the old "skills" there's always some lack of knowledge that needs to taken care of.
 
Me and my Companion uses two different computers on the recording, and the files he records for me always has a really high Hz sound in the background, no matter if its from the synth, guitar or vox. 
The sound is maybe in the 5-7 khz tone.
 
Any ideas?

could be a problem with his audio interface or his computer (e.g. poor power supply, interference from other electrical equipment)
 
you could try hooking your interface to his DAW and vice versa. should show you quickly which component causes it.

A high pitch sound can come from bad converters.
You should list your equipment in your signature  and while your at it, his system too, makes it easier to comment when trouble shooting. 

i used to get a noise that sounded like digital crickets chirping a few years ago & i was able to pinpoint the problem (after much toil) as the 64 bit drivers for the Tascam us-122l.  once i got rid of that device & replaced it with the Roland Quad-Capture, the terrible sound went away

I recognice this problem, had it with my Focusrite Saffire 6 USB. I think it disappeared when I changed my cables out from the soundcard from unbalanced to balanced. A friend thought it was interference from a radiator or armature close to the soundcard.

Yes, I had the same problem when I connected my new (to me) Crumar DMC-122 with the installed Gemini sound module that I had connected with TS unbalanced cables. Put TRS ends on (wires are star-quad) and the whine went away immediately. Using Focusrite 18i20.
 
It could be the OP's friend has something connected that needs to be balanced.
 
 

Leizer
I recognice this problem, had it with my Focusrite Saffire 6 USB. I think it disappeared when I changed my cables out from the soundcard from unbalanced to balanced. A friend thought it was interference from a radiator or armature close to the soundcard.

Always use balanced cables if possible, it could be anything from a bad ground to cables crossing each other. I used to start at the wall and trace to the comp box when I helped build studios. Balanced cables will solve 90%  gear noise and bad grounds.