PAXboy - assuming your friends are going on holiday, want reasonable certainty of catching their flight, want a reasonable amount of sleep on Friday night and don't want a high level of stress (passing through Luton's terminal on a Saturday morning in the July peak is already high stress), would they consider staying in one of the hotels near the airport ?
Ibis has a double room available on Friday night for 65 pounds, while the Holiday Inn wants 68 pounds