It is unlawful to pretend you have more than one card if it is a singleton. See Law 73 D
Right to Explanation of Bids
Players may not refuse to explain bids. They must explain clearly and answer all reasonable questions. See Law 20
Dealing with breaches of Law
Players should not self-regulate. If something goes wrong, they should always call the tournament director, or a senior player with knowledge of the Laws. It is unethical to object to the TD being called. See Law 9B.
If they do not call the TD, they are waiving their right to seek redress later.