Answer
Employers are generally expected to enforce their rules fairly and consistently.
If the member is the only one disciplined for something that is common practice, it may be that management is starting to crack down on that rule for everyone, and this member just happened to be the first one to get noticed.
You can ask if there was a recent warning issued that the rule would be more strictly enforced. You can ask the member to collect information about how widespread the common practice was.
If the member is being unfairly singled out, this might be grounds for challenging the discipline.