@Blackbird
Stupid how? Sure, she was there next to her desk when he arrived, but that is not condemning. Her excuse is valid (if she is the one who is actually behind it). Nothing would come from questioning her by himself since she can just lie, using her excuse of cleaning it up. Him leaving her be actually is the right choice for the moment. It forces her action. Either she does not clean it up, thus proving herself guilty of being the instigator, or at least in aggreance with then, or she cleans it up, thus temporarily "pausing" the bullying.
Either way it is a loss for her IF she is the instigator. If she is innocent, then no issue.