If he's considering two multi-touch devices that do much more than play music, I'm guessing he doesn't want a plain phone to listen to music on.
Both the iPod Touch and Zune HD are pretty good devices. It really depends on what you need out of a handheld device like this.
Keep in mind I haven't used a Zune HD in a long time, and I use an iPod Touch daily. I did do a lot of research for you though.
If you're going with the iPod Touch, it might not be a bad idea to wait until September when they usually announce and release new iPods.
Advantages of the iPod Touch
The iPod Touch actually has external volume controls, on both the device and the earbuds.
Bluetooth headphone support.
The touch's interface is a bit more consistent, straight forward, easy to understand and functional. It's not going to wow your friends, but it works great.
The iPod has iTunes for purchasing assloads of music, downloading podcasts, and accessing other media.
iPod Touch has the MASSIVE market dominating app store for access to more applications and games than you'll ever need. This blows away the Zune HD's itsy bitsy marketplace that nobody releases applications for.
The iPod Touch actually has an email application.
MUCH faster web browser. (Mobile IE lol)
Renders web pages faster when scrolling and zooming.
Larger display with higher resolution. (iPod: 320 × 480 px 3.5 in - Zune: 272 x 480 px 3.3 in)
You can actually have multiple pages/tabs open in the web browser. Not sure if Microsoft ever added them, but I can't find anything on the Zune HD's browser ever getting them.
Every time you leave the web browser in Zune then come back to it, it has to reload the page you were on. The iPod Touch keeps the web page in memory.
Applications open and close much faster on the iPod Touch.
Wider (in portrait) display, which makes a HUGE difference when browsing the web or using applications. You can notice this most when zooming into parts of web pages, as text is larger and more readable due to the space it has.
Multi-tasking.
Advantages of the Zune
OLED Screen.
I think it has a bit better audio quality.
Faster screen rotation.
HD output for watching movies on your TV.
Apple says their device gets 30 hours of audio and 6 hours of video, while MS says their device gets 33 hours of audio and 8.5 hours of video.
MS offers a deal where you can download (I think) an unlimited amount of songs for $15 a month and keep 10 at the end of the month.
FM Radio.
It's pretty.