Thetis opened her mouth, then closed it again. She knew that trying to hold Achilles back when he'd made up his mind was like trying to hold back the Aegean with a broom: nigh on impossible. At the same time, though, she knew there was a time and a place for this confrontation, and here was probably not the best one.
Weighing her options, though, would really take more time than she had, so she just sighed and signalled for the check. It was probably for the best that they got all of this over and out in the open, anyway.