If she has a British passport then she's a British citizen regardless of what the Iranians might think. If she's a British citizen she's entitled to all the protections that any other British citizen is entitled to.
Dual citizenship - GOV.UK
As a dual national you can’t get diplomatic help from the British government when you are in the other country where you hold citizenship.
For example, if you hold both British and Chinese citizenship you can’t get diplomatic help from the UK when you’re in China.