visiting the US with criminal record?

[_fortress]

           I live in the US and have a friend in Norway who would like to come visit for a few weeks.  He's afraid to apply for the US Visa because he was charged with possession of pot a few years ago.  I've tried googling, but I still don't quite understand how this stuff works.  Do you think he have trouble getting into the US for visitation purposes with one, minor criminal offense such as this?  (I consider it minor, anyway...)

Thanks for any info you have!