Other than this Alternet article, I wouldn't know what to do. From the article I would gather that your friend should find out if VA is one of the states cracking down on HOAs (p.3 of the article) and also see how the neighbors feel (maybe if there's enough of an outcry...). That your friend's first warning letter was received in the same envelope as the second might be of interest to a lawyer, but past that I have no idea.

Good luck. Your friend's garden sounds mouth-watering! Nothing compares to just-picked sugar snap peas.