If the blood was tested outside of the GP's surgery, they had to pay as they were in the UK as visitors. There is no free NHS treatment because a visitor is over 65.
If their country has a reciprocal visitor health agreement with the UK, that might mean they didn't have to pay, but that depends on what type of agreement their country holds with the UK.
You could pay it, fortunately that won't be much. Get the receipt and then they can (should) be honest in their visitor visa application. You should be able to claim that money back from the travel/health insurance they had.
Having an NHS number does not mean they get treatment for free. The NHS number is a unique number as it holds their medical record and treatment.