I'm working on a cake for tomorrow that should be refrigerated overnight... My concern however is that the exterior is a dried (hard) fondant overlay. Because it is a dried fondant exterior, do I still run the risk of the fondant sweating or being ruined from the moisture, or is it ok since it is already dried out? I've never done a cake like this before...

The refrigerator is not where the problem is. Refrigerators have dry, cold air. What I would do is crank the AC in the house to reduce the humidity when you pull the cake out. It may develop some condensation but that will dry off. Many people refrigerate fondant covered cakes with no issues.