The thing is... we know for a fact that he never thought the ring was worth £425. In fact, he most probably thought it was worth £500, £600, £700 or more - and he offered the price based on what he believed was much less than the true value, so that he could make a decent profit. For all we know, he may have offered her a lower price, with a higher margin for him, than he normally would if, say, it had been a ring bought new by somebody in her family that she still had the original receipt for.
He would doubtless have explained this away as "That's business" - and I wouldn't blame him, because that is the nature of business: you buy things for considerably less than you can sell them for.
When it comes to gold that's going to be sent away to be melted down, it's easy money for him, really - as with any of these "We buy gold!" companies. There's no need at all to use multiple skills to add value to a wholesale item and work hard at selling it retail to your buyer for a profit, nor to run the risk of being left with unsold stock; you're just passing it straight on to a big gold buyer, with a prescribed price that they WILL pay (for the correct grade of gold), purely relying on the fact that members of the public don't know how to get in touch with the big buyers themselves, nor to avoid being potentially ripped off by being 'assured' that valuable gold is just costume jewellery by the buyer.
He thought that he was going to make a very good deal on the ring, based on its true value; but it turns out that, on this occasion, the woman who sold it to him made a very good deal - one that HE initiated! As you might say, "That's business".