Why can't someone have their money restored if someone they paid in advance goes bankrupt? A friend paid a travel company €3000 for a holiday in Australia. However the travel company went bankrupt so now her €3000 is lost. She has been informed by the liquidator that the company had such debt that she will not be getting any of her money back.

To me she is a victim of a system that requires the customer to pay in advance - we get that you have to pay for a service, but if she paid in arrears like you would, say, a plumber, then she wouldn't have this problem. It seems unfair that some industries require advance payments and so present this problem even though the capitalist system is supposedly based on the idea you pay money for goods and services. She's down money but didn't receive the service.

So my (somewhat hypothetical) question is, why is it not feasible to just "restore" her money, if we assume it's undisputed that she has the right to it? The obvious answer is that since the company is bankrupt, there's nowhere for that money to come from, but why does it have to come from somewhere, isn't money just a representation of your current worth which you have to pay to receive goods and services? And since money is no longer tied to gold or anything - it would seem it's technically possible for the bank to simply add that money back to her balance.
That’s how bankruptcy law works. Every entity that is owed money - including firms that sold products to the bankrupt, workers - wait inline to get paid back according bankruptcy laws.

The state cannot make everyone whole, as that would just encourage fraud. E.g., create a shell company, get rich friends to lend it money, take the money out and declare bankruptcy. Your friends get their money back from the government, and you get their hands on the proceeds as well. You make it worth their while by passing them money back under the table.

The state could bail out certain groups, but that’s an entirely a judgement call by the legislative body.