Surfer,
Every professional has a duty of care, and even shop assistants must excersice care when offering opinions or advice where they may be perceived as having specialist or expert knowledge by the customer or receiver. However opinions and advice arec just that and the receiver is not obliged to follow that advice.
Regardless of above the offence of towing an illegal outfit lies at the feet of the driver. That offence is not reduced if the driver took bad advice. Unless the advice was malicious, or given in the execution of a fraud, the expert or adviser is not guilty of the driving offence.
If the car has a towing limit of 1500 and owner did buy a caravan with an MTPLM of 1590Kg, that is not illegal. It would only become illegal if the owner loads the trailer so its excedes the cars towing limit of 1500KG when hitched and on the road.
If an experts advice is wrong they may become liable if the user of their advice suffers a loss.
This ignores any other limit imposed by the drivers licence categories.