7.3 Conduct Penalties (Rule 21)

Misconduct includes rude behavior, offensive behavior, and aggression. Sanctions are applied progressively (Rule 21.1): Warning (yellow card): No point or service lost; recorded in the score sheet. Issued for minor unsportsmanlike conduct.; Penalty (red card): Opponent awarded a point and service...

Source: Rule 21.1, Rule 21.2

Send this to your friend — argument over.