Yes, I think after two forfeits it would be clear that this person is playing games and is not confident that they can win the debate. So it would only be right to declare the person who showed up for the two debates the winner. One time is okay, you can excuse that, but twice lets you know they are not prepared to debate.
In my opinion, debates should be judged on the merits of the arguments made and of the evidence provided. If someone is able to make a proper case, and successfully rebut their opponent in a short amount of time/words, their points should be judged on their own merits. Considering the DDO format, I would suggest that they should lose "Conduct," but if they somehow manage to have more compelling arguments, even considering that they forfeited two consecutive rounds, I see no reason they shouldn't be scored as having won "Arguments."