Seth used what little knowledge he had from the Marine Corps. The bullet passed through, so that was simple enough. Some of the bleeding had stopped, and color began to return to her face. But she was still getting worse.
Seth looked around, and then remembered his back up plan for a fight. If he was injured, he would inject himself with morphine, making him oblivious to it.
He know used that on her. Sticking the needle into her leg, and he shot the pain killer straight into her body, hoping it would somewhat stabilize her, and also hoping he wouldn't cause her to overdose.