From The Code in Friend at Court:
32. Disputes. Disputes over the score shall be resolved by using one of the following methods, which are listed in the order of preference:
• Count all points and games agreed upon by the players and replay only disputed points or games;
• If the players do not agree on the court in which the disputed point started, toss a coin to select the court.
• If the players do not agree on who served a disputed point in a tiebreak, toss a coin to select the server. (A coin toss may also be needed to determine the side in which the point is played and the end from which the server serves.)
• If the players do not agree on who served a disputed game, toss a coin to select the server.
• Play from a score mutually agreeable to all players;
• Spin a racket or toss a coin.