Here is a note from the Library of Congress:
Treatment: The Top Treasures are stored in a cold storage vault in the Conservation Division. Each treasure is housed in a protective enclosure. The lack of oxygen and the cold temperature and stable humidity inside of the vault extends the life of these documents by several hundred years. The temperature in the vault is 50 degrees F. The relative humidity is 50%. These are ideal conditions for paper documents.
http://www.loc.gov/preservation/cons...bbcmanus2.html
I am assuming he cannot regulate the temperature that well, and not sure how the humidity correlates. My best advice is to throw some silica packets in with his cards just to be safe. They are available in bulk on eBay and I believe Amazon. I use them in my firesafe just to be cautious.