One has a diplomatic authority to not allow them to have nuclear weapons by treaty.
The Nuclear Non-Proliferation Treaty.
By treaty, Iran is not allow to have nuclear weapons. They are not allowed to attempt to create nuclear weapons, nor are treaty signing nuclear armed nations allowed to sell them nuclear weapons. They can legally back out of said treaty, but until they do so, they are in violation of said treaty if they create or own nuclear weapons. Backing out of said treaty is a clear sign they have nuclear weapons...as it did with North Korea.
Thus I also question Saudi Arabia's stance on buying nuclear weapons if Iran makes their own. Because, by treaty, Saudi Arabia cannot own nuclear weapons, nor are treaty signing countries allowed to sell them any, nor help them make any. This of course means that Saudi Arabia would have to legally back out of the treaty and purchase nuclear arms from Pakastan, North Korea, India, or Israel (as Iran won't sell them any).