There are several possible, and likely, punishments if you get caught.
First of all, if you are caught, you are all but guaranteed to be disqualified from the event you're participating in on behalf of your friend. Impersonating another player would be "Lying to a tournament official" when you register for the event, and therefore fall under Cheating in the Tournament Rules.
The Head Judge still has the authority to issue rulings even if the situation is not covered in the tournament rules:
- GENERAL PHILOSOPHY
Only the Head Judge is authorized to issue penalties that deviate from these guidelines. The Head Judge may not deviate from this guide’s procedures except in significant and exceptional circumstances or a situation that has no applicable philosophy for guidance. Significant and exceptional circumstances are rare—a table collapses, a booster contains cards from a different set, etc. [..]
- ORGANIZED PLAY.
b. Rules and Enforcement. By participating in organized play, you expressly acknowledge and agree to comply with any official organized play and tournament rules (see rules for Magic: The Gathering and Kaijudo) updated and published by Wizards from time to time and incorporated herein by reference (see applicable game websites for current rules). Organized play events are adjudicated by members of the Wizards’ community who provide us with reports regarding play (win/lose records) as well as player behavior, conduct, and violations of tournament rules and the Code of Conduct, and recommendations regarding disciplinary actions. [..] You also agree that all disciplinary decisions regarding actual or alleged violations of tournament rules and the Code of Conduct are final, at Wizards sole discretion, and that Wizards has no liability to you whatsoever in regards to such investigations and decisions.
By impersonating another player, you are breaking the Wizards of the Coast's Code of Conduct, which explicitely states:
- Do not impersonate other persons, including fellow users or Wizards of the Coast staff, by screen name or self-representation, or attempt to obtain sensitive information from other members.
Therefore, it's entirely possible your and/or your friend's account might be temporarily or permanently banned.