"I will take her plate." Tristan volunteers before anyone else can. It's exactly what he wants to be doing. A chance to see the party, and a dignified excuse to leave in a moment without snubbing anyone. And to not show up at all would be to insinuate that he is hiding, or has reason to, which would be unfair to anyone who would suspect themselves the reason. If he was assigned to making merry right now, then he would have to do it to the very best of his abilities. And he would! He would. But there is a yawning chasm of difference between being happy for others, and trying to bring happiness to others. The first is far more taxing. 

Carrying a meal is a simple duty that he can do well and be satisfied with. And if Constance doesn't want to eat, then he will be content to not eat with her. Or, if she prefers to be alone, to take the chance to go for a walk and see if he can't find someone else unsuited to festive cheer in these circumstances, someone who feels lonely.

He wonders what it would mean if Sir Coilleghille will be one.