Insurance policies normally exclude cover where 'gross negligence' is involved.
For example, if you left your keys in the ignition, an insurer almost certainly wouldn't pay out if your car was stolen.
Similarly, an insurer wouldn't pay out on a fire dame claim if you decided to have a bonfire in your living room.
However an insurer should pay out when there's been nothing more than a bit of carelessness (such as forgetting that you'd left the chip pan on a lit hob).
The borderline between simple 'carelessness' and 'negligence' is a grey area. An insurer might try to argue that the person had been negligent but it's definitely worth submitting a claim and hassling them to pay out.