If these are neighborhood assocation regulations as opposed to actual city or county bylaws, it might be harder to fight, since your friend probably signed an agreement to obey the regulations when he moved in.

The first thing your friend needs to do is file an appeal, or if there is no appeals process, file something in small claims court, anything to get a legal stay imposed so he doesn't have to tear up his food in the middle of the growing season.  He can find ways to delay at least until harvest.  He can use a technicality if neccessary -- he didn't get the original letter, for one thing.  In most districts, if they don't send the notification by registered mail, it won't hold up in court anyway.

Second, he can nitpick them to death. If the regulations say no veggies and no herbs, he doesn't have to tear up his tomatoes because they're technically a berry and therefore a fruit.  Containered plants can probably be considered separate from "the garden" and could also stay.  If the regulations say "no food" or are more broadly worded so as to really imply no edible items can be grown, he needs to case the council members' houses and find out if any of them are growing rosemary hedges, or nasturtiums or other edible flowers.

Third, he probably has grounds to challenge the regulations on their face.  Assuming this is Raleigh, NC, he needs to call up the law schools at Duke and UNC Chapel Hill and see if they have students around in law clinics over the summer who could advise him for free.   I think the best bet is probably the Environmental Law and Policy clinic at Duke, but there's also the Community Enterprise clinic there.  UNC has a Civil Legal Assitance Clinic too.  Unfortunately, most of the clinics run only during the academic year, and I'm not sure whether they take direct clients without referrals anyway, but they might have some kind of staff around to answer questions during the summer.  Can't hurt to talk to them, anyway.

More resources:  
North Carolina Justice Center
Legal Aid of North Carolina
LawHelp.org/NC

Just a couple of ideas to start off with.  There are probably more avenues to pursue, including simple public opinion -- appealing to his neighbors.  If the garden's pretty and useful, and he gets compliments on it, the odds are that somebody complained because of his bumper stickers (to state the obvious), but other neighbors could well be outraged if they found out he was being forced to tear it up.  He could put a small sign in the garden saying that the neighborhood assiociation is trying to force him to dig it up, and asking for support.  Maybe post a petition and/or the phone number and address of the responsible parties.  Again, it isn't likely to hurt, and could help a lot.