Curative magic, surely. Curaga, Full-Life or something. Notice that neither Seifer nor Squall needs stitches on their head wounds at the start? They're both fine within hours. Some kind of magic would've been used to aid the healing process. Edea's magic is even stronger, since she's a sorceress, so her healing would be greater too. Healing Squall's wound would leave him alive and well for interrogation.

Besides, we all know that getting stabbed in the shoulder by your nemesis is the key to survival in Final Fantasy. Just ask Cloud.