Congressional Subpoenas are at same level as Court Subpoena. So if Congress subpoenaed her documents, she could (willingly) turn them over and not be in breach of her NDA.
Congressional hearings can compel witnesses to answer questions under oath, among other things.
People brought before a Congressional hearing like this can be held in contempt of court for failure to cooperate. Testimony is subject to perjury laws.
This wasn't just someone writing a letter to Congress with some claims. This was a Senate hearing convened on the matter.