Hermione missing Arithmancy is what first clued her in. An hour later saw her standing in the hospital-wing.

"You saved his life."

"...I did?"

Pomfrey nods at the bed-ridden Ron. "Professor Slughorn didn't have time to brew an antidote, luckily I still recalled our conversation on basilisk venom."

"You vanished his blood?"

A nod.

"...because Harry didn't give him a Bezoar?"

A sharp inhale had her turning around to lock eyes with a distraught Harry.

"I-I forgot."

Suddenly she was reminded of a certain textbook. She paled.

"You saved his life."

"No. I didn't."