1:28 p.m. on April 7, 2011 (EDT)
I've been a vegetarian for years, though I'm only a partial one now. I've never been vegan.
First, I'd find out what your friend likes to eat and then bring along a variety of that stuff that's safe and packable on a hike. After all, we all like what we like.
There should be plenty of options, just think of all the fresh and dried fruits and veggies, nuts, oatmeal, pastas, rice, and so on out there.
In the protein department, there's Justin's Nut Butters, among other options: http://www.trailspace.com/gear/justins/ (I haven't tried these, but saw them the other day and thought they sounded good.)
If it's only a hike, it shouldn't be too much trouble. I mean he eats every day anyway, right? You can pack along hiking versions of what he might have eaten anyway.
If it's overnight you'll need to plan better, but it's still very doable.
I'd recommend he bring his own food, since you may not be aware of which versions of food are or aren't vegan-friendly. Sometimes it's hard to tell with hidden ingredients. Once you know what he likes, you can suggest some options and how much he should bring along. Go for a variety, to see what he likes and what works on the trail.
I just saw an article on vegan travel and backpacking:
http://wideworldmag.com/howto/backpack-vegan
Also see:
http://www.the-ultralight-site.com/vegan-backpacking-food.html
http://www.mocs1986.com/2010/01/vegan-backpacking-menu.html
http://www.livestrong.com/article/272827-vegan-backpacking-food/
http://outdoorherbivore.com/ (the site's not working now though)
Hope that helps. Let us know how it goes.