I think the problem you are going to find is that (IMO) optimal headlamps are not available.
First off, for serious night hiking, I think I'd avoid the AAA lights. I think I'd shoot for a dual brightness (or more) light based on at least three AAs. Probably run lithiums to lighten up a bit, both on the head and lugging lamp fuel. You also probably want a full width band with a top strap for comfort and the ability to have the lamp stay put.
The second issue is how much light is he going to need. I think three 5MM LEDs is probably going to be OK for trail work. However, if he needs more light things are going to be a bit more difficult.
If we assume that three LEDs are OK for what he is going to do, I'd say look at the Princeton Tec Yukon (3AA), Petzl Myo 3 (4AA) or Myo 5 (4AA). It's possible the Black Diamond Gemini might be OK, but it only has two LEDs. I'd perfer the Yukon HL which has a Luxeon Star for a main bulb, but I don't think you can get them yet.
While I looked at them all in the store, I only have the Yukon. The Myo seems overly complicated (and costly) and a bit gimicky. The Yukon is simple, to the point and obviously built to be sturdy and waterproof. If I was going to get a Myro, I'd pick the Myo 5 since it has the extra LEDs. Both the Yukon and the Myo seem to be built well.
Things to consider tho - the Yukon only has three modes - off, on-main, on-LED. I think the Myo (at least the Myo 5) has more modes. I think the modes are more trouble than they are worth, but YMMV.
If he needs more light, you probably want to look at a regulated light. The Black Diamond Super Nova comes to mind. I think uses Willie Hunt's LVR for regulation. I think the regulation provides brightness levels (edit: yes, I found it - three levels) so you probably can't go too wrong.
An interesting alternative might be the NiteRider Storm Helmet H.I.D. light with the optinal Explorer Headband (or the helmet mount). They claim 4 hours with that killer HID light, but in the end I suspect this isn't real practical, especially using rechargable batteries. I'd be very curious to see how comfortable it is - anyone try this?
The HDSSystems Actionlight is potentially a good fit, but hasn't been available for well over a year. They claim to be available in Dec.
While several people have liked it, I think the Petzl Duo light looks very poorly built and designed. Before the current crop to dual output lights showed up I went and checked it out in the store, fully expecting to buy one. I was so disappointed I decided to wait until something else came along (and that's pretty hard for a flashaholic)).
-john
Extra comments about the Yukon:
I like the way the lamp assembly is minimal and pivets up and down. Makes it easy to grab it and move the light where you need it.
LEDs are pretty blueish.
Main beam is focusable, but pretty splotch at wider settings. I leave mine focused pretty narrow because of this. The Yukon HL would take care of this issue.
Yukon Pics