If the FI wasn't happy with the standard, then he/she is quite correct in not signing - as the CAA have said. 'Irrespective of standard' would be pointless; the dual training flight is nothing more than a bi-ennial Club check. If your Club member wasn't up to scratch, then perhaps further training will sort that out and she can have her logbook signed. Alternatively, a revalidation proficiency check may be a cheaper option if she prefers.