hi - looks to have been an attractive piece, sorry to hear of the mishap. Firstly, just to advise that the GMB doesn't discuss values - the reason for this is brought home by the very situation that you now face - namely the contradiction of opinions between yourself and the customer.
The only way of remedying this, for you, is to locate an identical piece, which might help you to arrive at a mutually agreed figure ........... unfortunately, as you've already commented this is probably impossible (finding an identical piece).
Regret your picture is too small and lacks evidence of the sort of detail that might help to assess whether this piece is late C19 or C20, presumably you are unaware of the genuine age of this piece - might your customer have indicated something here?
Spatter glass has a longish life span, and appears during the C19 and lasts in some form or another until the late 1930s - it was a not uncommon form of glass decoration, especially in Czechoslovakia from c. 1920 to the mid 1940s.
I have a feeling that we aren't going to be able to help, particularly as this piece is now lost and additional pix are therefore impossible to source.
It would be assumed by most people that if this item had been of substantial monetary value as the customer appears to have suggested, then the customer would have had an equally substantial insured value.