Here is a comment I found in the SimpleGraphAdapter code:
// For now, support only directed graphs. This may get modified in the
// future to support undirected graphs as well, at which time some
// mechanism must be added to tell this class the directedness of
// the graph that is about to be created. Possible solution: Add a
// SimpleGraphAdapter.Directedness property.
So that is a "to-do" item that hasn't been done yet.
You could do one of the following:
1. Implement your own adapter from scratch.
2. Copy the SimpleGraphAdapter code, rename the class, and adapt it to your needs. I believe the only changes required are to change the GraphDirectedness argument to the internal "new Graph()" call, and to change the oEdges.Add() call to use a third
argument of false. If you go this route, the source code is available as a download on the Downloads tab on CodePlex. You don't need to rebuild the source code; just steal the SimpleGraphAdapter.cs file.
3. Use the SimpleGraphAdapter as is, then copy the vertices and edges to a new undirected graph and discard the first graph. That's what I would do, assuming I had enough memory.