In a medium sized saucepan, simmer the rhubarb, sugar and water over moderate heat, stirring occasionally, for 15 minutes, or until the rhubarb is soft.
Remove the pan from the heat.
Drain the rhubarb and transfer it to a large mixing bowl.
Discard the juice.
Set the rhubarb aside to cool.
When it is cool, stir in the mashed bananas and the double [heavy] cream.
In a small mixing bowl, beat the egg whites with a wire whisk or rotary beater until they form stiff peaks.
With a metal spoon, fold the egg whites into the rhubarb mixture.
Spoon the mixture into a decorative medium sized serving dish and place the dish in the refrigerator.
Leave to chill for at least 4 hours before serving.