She said she was buying it for her Iranian friend - that's 
  enough to reasonably suspect.

No, that's reason to inform them that they are not allowed to do that. Tell them they would be breaking the law if they did that and could go to jail. If they insist on buying the item, it is out of your hands. If they still explicitly acknowledge they are intent on breaking the law, you may be required to inform the feds, but that's it.