Check that he is reporting each and every time that he is playing. Ideally he should be writing the gambling in an Excel or Googledocs spreadsheet. Every one of them for at least three months.
A common problem is that the people report only the good events and forget the bad strikes. Other problem is partial compensations: "I won +$50 and +$10$ and I only lost once -$30." really means "The firs week I won +$50. The second weak y lose -$40, but it not a lose because they come from the money I had won in the first weak. The third weak I won again +$10. I only was unlucky in the fourth weak because I lose -$30."
It's not voluntary lying. Lucky strikes are interesting anecdotes, unlucky strikes are expected and boring. And bad memory and "compensations" are normal human bias. So write down everything systematically, or it's not useful to make statistics.