This is a very complicated question and does not have a simple answer. It depends on whether the interior of the walls (including the pipes) was considered common area or not. I would advise that you contact an experienced condominium attorney, bring your condo docs, including original plans, and they will be able to advise you. Good luck to you.
As Atty. Amarantes advised, the key is what the condo docs say. Most common is that pipes within the walls are common area, and thus the responsibility of the association. If the owner having a medical issue allegedly caused by this actually makes a claim, it should be referred to the condo's insurer to challenhe and/or handle.
To questioners from West Virginia & New York: Although I am licensed to practice in your state, I practice on a day-to-day basis in Massachusetts. I answer questions in your state in areas of the law in which I practice, and in which I feel comfortable trying to offer you assistance based on my knowledge of specific statutes in your state and/or general principles applicable in all states. It is always best, however, to work with attorneys and court personnel in your own area to deal with specific problems and factual situations.