You're baiting the mechanic.
If an old guy turns up with middle aged daughter they'll assume she's there to help and deal with the daughter. If somebody has gone to the lengths of bringing a friend to the MOT (it's not somewhere you go for a date after all) it's probably because they're wanting help.
If she turned up by herself they would assume a more general knowledge of cars, which is still far below what she has but wouldn't be dismissive.