Absent Minded Partner

The correct answer is:

Abby’s opponents must forfeit the point since they should’ve both been paying attention. 

From ITF Rule 21:

The rules states that “the server shall not serve until the receiver is ready.” Since the opponent who was receiving the serve indicated to Abby that she was ready, Abby had every right to serve, and since her serve was good, her team wins the point. Whether or not the receiver’s partner was paying attention is irrelevant.