If it was a youngish lad in Calis Sunday market he's normally very good - I have bought several game selections from him in the past and have recommended him to others who have had no problems.
I recall our games refused to work after a while and (forgive me, this is 5+ years ago) I researched it and found that it was remedied by making sure the DS time and date was correct. I can't find the info now and I can't explain why that would work but it did for us. Doesn't mean it would work for friend's grandson but a fairly easy thing to try.