I'd be quite-surprised if the owner of the gallery does not have general business insurance, including the contents of the building they're operating in.
I've never been able to lease a commercial location without having at least commercial liability coverage in a pretty sizable amount ($1m is not an uncommon requirement at all) and contents coverage is generally part of that policy and not particularly expensive. Where it might get sticky with the gallery owner is if there are irreplaceable items on display (e.g. original paintings) but the owners of those are unlikely to let her have them for display without knowing that they're covered if the building burns to ash. For irreplaceable items they are always scheduled individually but this is not the case here -- the exposure is quite ordinary and commercially-reasonable.
I think inquiring of such and expecting it is a pretty-ordinary request and one that I can't imagine anyone with high-value original artwork would not be asking about prior to allowing her to display their work.

