Mikoto tenses, frowning, wondering what to do about this...

Regulus already has an idea. The confundus charm, depending on how it's used, can confuse a target and - with some effort on the caster's part - even convince them of things. In this case, Regulus thinks very hard about how there isn't anyone there and everything's as it should be, as he silently casts the spell on the Andalite leaning into the shaft below.

((Spell info here:
http://harrypotter.wikia.com/wiki/Confundus_Charm ))