Again, they had been on since Piccadilly and hadn't invested in a return ticket on their way out, if they had bought a ticket at all.....
I haven't been in to Piccadilly since 1st April, are there still RPIs at the doors to platforms 10/11? If they are that usually gets the majority of ticketless passengers on Chester-Manchester services who stay on to Piccadilly. If they try and claim they traveled from Levenshulme-Piccadilly and traveled from Stockport instead of Piccadilly on their return journey then they might actually be paying more than by buying a return between their origin and Piccadilly.