Just for answering:
1. Yes, the mesh has only 1 side for materials and I don't have access to it outside from SL. So I cannot modify it.
2. Wearing the mesh only makes the front part visible, yes. It fixes the issue but since it's a long dress I cannot see the other side between the avatar legs (for example)
3. The mesh has the inside faces so it's not an issue done by its normals
Help!