Beavertail Bother

I have a client who operates a 3.5 T beavertail on hire and reward work. Does the fitment of a towbar to the vehicle mean that he has to have an ‘O’ licence and adhere to EC drivers hours rules etc etc because the GTW is now over the 3.5 T weight break? The beavertail does not qualify as a ‘dual purpose’ vehicle.