Cities are not in the business of sending around fence inspection teams, and as long as no one complains, a nonconforming fence may stand forever.
Tell the neighbor about the law as soon as possible. She probably doesn't know what the law is, and if the fence is still being built, may be able to modify it at a low cost. If she suggests that you mind your own business, alert the city. All it takes in most circumstances is a phone call to the planning or zoning department or the city attorney's office. The neighbor will be ordered to conform; if she doesn't, the city can fine her and even sue.