your friend did not find a link to the “secret” vatican archives, but he may have discovered the link to the Vatican Archives catalogs, and the documents that have so far been made available on line, which is on many Catholic websites, including vatican.va. the vast portion of the Vatican archives are open to serious researchers, and the catalogs are available on line, although only a portion of the holdings may actually be viewed on-line.
The “secret” archives are those that deal with matters that must remain private or “occult” (hidden) such as matters of the internal forum (conscience), marriage tribunal records, canon law proceedings against priests, Vatican administration, personnel matters, etc.,and the personal papers of popes which remain closed for at least 50 years after death. This is standard archival practice.
Also collections are not made available to researchers until they are inventoried, cataloged and preservation needs have been met. Catalogs and manuscript descriptions for items in English are available in English, but the vast majority of holdings will be in Latin, and after that, in Italian.