Remember that adding weight anywhere on a car will not only give you more traction on the wheels where the weight is added, but also more weight for the tyres to try and stop or keep from skidding. That weight on a RWD car might make it less likely to get stuck and might make it more likely to loose control.
Get your mother a set of four winter tyres with wheels so she can be safer FWD or RWD. They will also help keep the car on the road and to stop better. They would make a great early Christmas present.