What does your solicitor colleague think about Asus liability to a party they've not entered into any form of contract with?
That's ultimately the catch point here - they have an ambiguously worded website (but you
can buy this laptop with 8GB installed, so it is not false advertising by any means) and the phone operative made a mistake about it being user upgradable - BUT, he didn't buy from Asus, he entered into no contract with Asus, he exchanged no money with Asus.
He should have confirmed this with his retailer. He did not.
All this stuff about 'not fit for purpose' is nonsense as I see it, it IS fit for purpose, it's a laptop and functions as a laptop, just not one very well equipped to deal with the specific task you have which requires 8GB RAM. If you wanted confirmation it was well suited for this particular purpose, then again, should have asked the retailer.
The retailer has done absolutely nothing wrong, they were asked to supply an Asus UX31A and that's what they supplied - it's not their fault the customer was misinformed by someone else as the what a UX31A was actually capable of.
If this retailer is someone your business use regularly I imagine eventually they'll relent for fear of losing future business but if they're not your regular supplier then I suspect they'll stand by their guns.
Quite what anyone would expect Trading Standards to do I don't know? Force Asus to refund the cost of a laptop that they didn't even sell because a member of staff made a mistake?