Compensation by negligent rebooking of an airline (Europe)

A flight originating in a small city of the European Union, with one connection in a hub (Airport A) also in the European Union, and with a destination on another continent, was canceled and automatically rebooked as follows:

• the new flight has a delay of one day (29 hours in fact)

• the new flight is run by another airline (somehow associated to the first), so it does not pass though the airport hub of the original company, but by that of the second (Airport B)

The reason leading to rebooking of the flight was bad weather in Airport A the day of the flight -- but not in Airport B. Crucially:

• The flight through Airport B still existed for the original day, i.e. they were selling tickets (the price online with that second company was ~4200 Euros, but I guess this is not essential).

• second connection flight (>6000 km) was not canceled, and the waiting time was originally about 4 hours in Airport A. There were other flights available that would have made it (so would have done a train connection substituting the first flight).

I observe that the airline responsible for the flight on the original day didn't make an effort to send the customer that very day (and could, since there were seats). The 600 Euro compensation rule for big dealys of European flights does not seem to apply here, since the airline can protect itself by claiming "bad-weather conditions".

Assuming this is a loss for the customer, what can she/he do?