To be fair, we see in 316 that the witch is standing over or very near the mirror too (panel 3).
Maybe they saw something we didn't.
But yeah, the witch was clearly counting on "gloating is a free action" ^^;
@panaru That or he wanted to be the "savior" by bringing back the book at the right time... but an accident happens before. Still makes him an dumbass (to be polite), though ^^;