A friend asked me to be a witness at his marriage across the border without polygyny permission. Should I agree?

It is better to refuse, because being a witness means you are helping a marriage that avoids the procedure that protects his wife. In some states, those who abet can also face action, and you may be dragged in if the case later goes to court. The principle is clear: “help ye not one another in sin and rancour” (Surah al-Ma'idah 5:2). Refuse as a friend, for example, ‘I value our friendship, and that is why I can't be involved.’ Then encourage him to take the right path: be honest with his wife and apply to the Syariah Court. That is a friend's real help. See marrying in Thailand without permission.