He'd never seen the effects of silver poisoning before. It was gross, to say the least. Come to think of it, he wasn't even sure if silver affected Moony in the same way; he'd never wanted to try it, and most information about werewolves was rather hit-or-miss.
This was another reason he should have brought the others, because he'd only thought of one solution, and he didn't like it. On the other hand, what other choice did he have? Beth was clearly dying, and it wasn't as if he could take her to a Healer (or even a Muggle doctor).
"Beth?" Peter removed his hand from her wound. "Try not to move." He took out his wand and pointed it at her stomach. "And please, please don't die." Taking a deep breath, he swallowed his fear and shouted, "Accio Silver!"