A close friend (who is not the type to use Stack Exchange) and his family hope to soon enter Canada at the land border to visit my family. He and his family (wife and child) are US citizens. However, 15 years ago, when he was in his early 20s, he was denied entry to Canada because the border agents found cannabis in his car. It was a small amount of cannabis that he forgot was there in the car - and certainly not intentional trafficking. He was refused entry but not charged, and he has no other convictions in the US or elsewhere.
What level of concern should he and I have? What is the likelihood that he will be denied entry when he attempts to drive in with his family? Is there anything we can do beforehand to help his chances? Any specific approaches to dealing with the border agents(s)? For example, we are unsure if he should preemptively raise the issue with the gate agent versus just letting them ask their usual questions.