- Try to export your file with the 2013 and 2014 version of fbx (the 2014 wont work correctly, but probably it will somehow solve the problem)
- So you already flipped the normals?
- Could you probably post your step by step exporting process?
- Import the obj file that you have sent to your friend into maya and without doing anything with the mesh, just export and import it into the UE4