Here's the scoop. ND CCW permit holders can give their card out at the gun store and not have to have the NICS background check performed. The CCW is photocopied, appropriate section filled out in the NICS form and the photo copy is attached to the form. Now, that being said, it may be a requirement or policy of the store to run a background check on all purchasers of firearms, regardless of CCW. The other issue is it probably depends on what your friend is on probation for. ND CCW maybe allows or doesn't restrict applicants with certain misdemeanors on their record. A felony...well thats a different deal.
As for your buddy not knowing when his probation was complete is a line of milarky. Anybody who has been sentenced to time or probation knows when it is completed. Personally that was a good catch by the salesman who performed the screening whether it was store mandated or he just made the call out of force of habit. People used to come in to try and purchase guns at our store all the time and try to pull sketchy stuff and then pleaded ignorance when we sniffed out what was really going on. Anyway, when your buddy gets off probation I hope that he can get the gun he wanted. Until that time he should probably follow the laws and parameters of his probation to keep himself from getting into further trouble.