Neville missed most of what she said, but he got the hint that she didn't want him grabbing at her ankle. He wanted to protest but didn't have the words or the time to explain it. If this basilisk was like the one from his world, her vision would start to fade soon so he couldn't even spell things out. He did, however, grab at her shoulders and press until she sat back down. Moving wouldn't help, even if she could stand.
When she seemed to reach for his wand he paused only briefly before lighting it again. He wasn't sure that was what she wanted, but he thought it might be. And if it wasn't, he would be able to understand her more easily when he could see her better.
Once lit up though, he could see she was in trouble. She looked pale and sickly, sweat shone on her face but her hand on his was cold. He had an idea. And he didn't know if it would work, but the other option was just letting her die anyway so he reached over, took hold of her hand with his other one, and moved his wand hand away.
He mouthed I'm sorry., pointed his wand at her chest, and cast a statue curse, petrifying her quickly, before the poison could take any further effect.