So personal and revealing a situation; I'll tag on to previous comments:
- Depends on the nature of your relationship; how close you both are.
- Depends if he's aspiring to be an audiophile.
- Depends on if he's asking for your "honest evaluation."
I usually just let my set sound for itself. I'd invite him over for listening and let him hear what you perceive as a great system. If a difference is obvious to him; he'll notice and say something. But it could also be his ability to discern music and sound is so different from yours he prefers a bright or boosted sound to details and refinement.
Funny that's just how I got into audio; a neighbor came by I showed him my new Sony and Boston Acoustic system with speakers buried in a bookshelf. He kindly invited me over for a listen to his "audiophile" tube system with electrostatic speakers. In those 30 minutes, my mind was blown at the steep difference between his sound and mine. I immediately began my plotting to refine my system, approach and knowledge because I now knew what I was missing.
Or you can just say, "That's an interesting system," and smile.