Was the CD that you played in his system original or burned? And if burned, was it from MP3'*? If you play burned CD'* (from MP3'*), and if the MP3'* aren't best of quality, they will sound a little weird in a system that normally sounds good.
I've noticed this quite a bit, and so as a guideline, I tell people to rip their MP3'* at no less than 196k 44khz at not at a VBR (Variable Bit Rate). 128k MP3'* that come from the net can be touchy.
Recap: head units sound excellent with excellent music sources. So if it doesn't sound up to par with other music you've played, make sure the source is high quality, and when burning CD'*, try to copy them from the original (or if you're burning an MP3 CD, rip the original CD at no less than 196k as stated above).
'95 SSEi, few electronic mods (and a stupid coupler)
HU (Sony MDX-CA680, European Ver; Sony 4x3-ways; 91, 93dB)
Yup, that'* ISU