I'm confused by some of these complicated replies.
If i'm reading things right the OP built a home server for a friend. Said friend needs to access different content (i.e. video, audio, etc.). Does he go with an external box to a receiver or does he go with a receiver that has the built in ability to stream.
Without hesitation go with an ATV2. Jailbreak it, install XBMC and you're golden.
Plays gapless 24bit audio (wave or FLAC), 24bit multichannel FLAC is coming with the next major XBMC release this December (already in the nighty builds). Damn near any video format you want to throw at it it will play. Use it wireless or wired. Pass-through audio.
For $99 (ATV2) you just can't go wrong. The only thing you are limited to is true 1080p of which the ATV2 does not do due to hardware limitations. Although I have played 1080p .mkv files and they work fine.
Go either HDMI out or optical if audio only to whatever receiver you have. Done.