How does prosecution for fare evasion on london underground work?

A friend - really not me! - was caught on a tube train without having swiped his oyster card on the way in to the station. He was actually in a zone covered by the travelcard on his oyster, but as he lived for a while i a place with no oyster machine, he had lots of unfinished journeys on it. This was months ago. Now he has received a letter from TFL. Is he getting a prosecution notice? I haven't opened his letter and he has moved out now, but I can make out the details that it is a piece of paper he has to return to the broadway office within 14 days. to an office which has the word "warnings" in it's title, btu I can't make out any more. Does anyone know what this letter is? Is he getting off with a warning, or is he being prosecuted? Thanks.