First of all - if she stated to them at start of policy she had a large amount of gold - they should have asked how much - and then that amount would have been covered on the policy.
Were there are VSI - valuables sum insured - noted on her policy?
if she had £16,000 of gold in the house - there would have been all sorts of T&C on the policy, covering security etc
Unfort her nationality is no excuse, but when the policy was set up by a broker?? - it should have been made clear to her what was covered and to what extent