What is a betting code? Perhaps a betting slip that is shared within the group?

I don't understand why your friend was betting only on the betting slips that were shared and why he didn't make his own betting slips, maybe it could be that the group is not a legitimate group.

Then the scenario you described is a bit strange, because when your friend consistently uses it, he loses and when he leaves others, he wins.
I can't think of anything to do with betting like this.