I think he did actively but subtly drained her MP, not with an actual spell but more like his compression and absorption technique used during the exam.
Page 28. "Tremendous Growth" indeed.
Rina's waist-to-hips ratio is pretty incredible.
She is no match for Trace-chan, but lawdies... that girl has some heft in the hips. Another few years and she'll have to turn sideways to get through doors.
Honestly, I expected him to go straight for the archmage to distract her and thus dispel the fireball, but never in a hundred years would I have thought that he just waited her out.
Legend.