Goldenbolls, here is a typical Santander policy, please don't assume that it is the latest version but it will give you an idea
http://www.abbeyforin...om/library/lib_18.pdf
I can't see it mentioning keys in the lock but it does show £1000 as the single item ceiling (but bear in mind this may not be exact the same policy as your ex's).
Another caveat though is that if the total of the jewellery value is a lot, the insurance company can look at that as a proportion of the total contents sum insured, and if proportionately it's high, they can say that she is underinsured altogethe,r and only pay out a proportion of the total claim. This happened to my mum with another company, she hadn't reviewed her contents sum insured for years so they would only pay out a portion of the value of the stuff which was stolen.
I hope this helps - IMO always worth keeping evidence of the value of anything worth a few bob, but I realise that this is too late to help your ex this time.