Male/female work relationships with not just an imbalance of power but also an imbalance in age need boundaries.
Ask him what those boundaries are?
Because he's starting to act like a dad outside the work place, by getting involved with her failing her driving test and offering her lifts.
Tell him to act like a manager - and if she needs a driving test, get the company to sponsor more driving lessons or a repeat test.
If the company is not willing to do that - he's crossed a line.
She's a teenager. All teenagers like to push boundaries. Equally all teenagers are vulnerable to people in power.
Ask partner how he would feel if a man of his age stepped in and started giving 1:1 lifts to his daughter to school.
Then tell him to grow up, get some perspective and start acting like a manager.
This girl almost certainly would not. But ask him if she made a complaint to the police about unwanted sexual advances or assault - how would he defend himself to police? How would he justify alone time with her? He's making himself unnecessarily vulnerable - by being unprofessional.
Are his actions legitimate business manager actions - or is he straying into grey territory without boundaries?
Tell him to grow up, get wiser and be more professional. Because eventually, maybe not with this person, his unprofessionalism will bite him in the backside.
And if a female employee (any future female employee of any age) makes a complaint about him. He's already got a reputation of spending unnecessary 1:1 time outside the work office with a teenage female employee.