To Ron Thomas and Beth Seifart, the poster is asking if he's guilty of interfering with the sale of his neighbor's home. No need to insult him by implying that he was less than diligent when he purchased his home. His agent, however, should be flogged for not noticing the PB pipes and warning him of the possible future problems.
That said, many homes still have PB pipes and have suffered no consequences. Whether or not it makes sense to replace them before or after a problem occurs is up to the homeowner.
I don't see anything wrong in what you did....especially if this is an issue that should be disclosed.
Now, this isn't a legal answer, but I don't think this is a legal issue, either.
You didn't tell any tales, or start any (unfounded) rumors - you simply suggested the buyers ask the sellers a question based on your personal experience.
If the neighbor was trying to withhold that information - shame on them.
Just my 2 cents!