Excellent tips everyone. They're only using Ryanair because they have no other option from Stansted.
The bit about boarding passes is one area of concern. Presumably you download and print the boarding passes for the outbound and return flights at the same time and take both copies with you? Halcyon - what did you mean about entering passport details?
As regards the hand luggage sizing, does the bag itslef have to be within those sizes or it is just the case that the bag and its contents has to be able to fit into a container of that size? They're both wanting to take soft holdall type bags for their hand luggage as opposed to more rigid bags which are heavier.
