Check out the piercing info on kellymom (http://www.kellymom.com/bf/concerns/...p-mother.html).
Basically, nipple piercing shouldn't impact breastfeeding. Several of my friends have nipple piercings and while several of them removed their piercings for breastfeeding not all of them did...but none of them have had any problems. One did have her doc tell her that she might be more prone to thrush or infection, but that was it.
Piercing can cause scarring though (and more scarring if there is an infection) so that can be a concern...if your friend is prone to scarring she may want to reconsider. Oh, and there may be some wild spraying at first...one of my friends sprays almost straight up when her dc pulls off since the piercing hole kind of smooshes the flow of milk to one side!