I agree, I am not trying to be argumentative, but I am sorry, you can not tell if a guitar is going to lose it's tune in 15 - 20 minutes of playing. Some may be fine for an hour, then next time you pick it up it has faulted, I am not on the side of buyer or seller, just think it could be a problem the seller was NOT aware of as they admitted to not playing it themselves for longer than 5 minutes at a time. Face it, who knows? who thinks? it is the OP's call, I was just saying for the sake of saving themselves a neg - I (personally) would refund - less said charges etc.

Anyone who plays a stringed instrument, and has a musical ear, will tune the instrument, every time they play it.Whether that is daily or once a week.

New strings, require tuning more often. 

If the buyer believes they should be able to tune it and it stay in tune.........they're kidding themselves.

If the buyers experience is already a negative one, a refund will not make a difference.

Music lesson would probably serve them better than a refund. Or an electronic tuner!:-)