I think we can safely say that the claim of having simply 'forgotten' a borrowed £500 item and left it in the hotel room (and who doesn't have a good check around everywhere just before leaving?) is, erm, lacking in veracity.
I'd understand her nonchalance towards looking after expensive goods if she were stinking loaded (and spoilt); but if that were the case, she would have bought one herself and/or pinged you the money for a replacement straightaway without even registering it. However, she is pleading poverty!
All circumstantial evidence is pointing to a phenomenally high likelihood that she simply must have decided to keep it for herself, or sell it to cash in.
The thing with either of those scenarios, though, is that, absent the original 'mysteriously' turning up, when she does eventually repay the £500 (whether willingly or not), all she will have done is to spend £500 on a used item that she now has to keep/sold for undoubtedly well less than £500 - when she could just have legitimately got herself a brand new one, without any of this awkwardness or dishonesty, for £400 (and a boatload of Advantage points) on offer from Boots!