Just to add, here, Fenrir is about to attack Lavender. He doesn't actually get close enough to bite her.
This is the actual text.
Two bodies fell from the balcony overhead as they reached the ground and a grey blur that Harry took for an animal sped four-legged across the hall to sink its teeth into one of the fallen.
'No!' shrieked Hermione, and with a deafening blast from her wand Fenrir Greyback was thrown backwards from the feebly stirring body of Lavender Brown.
I think Lavender survived the battle. My reasoning is that JK would have said in the book if she'd died. She would have had Ron looking at her body or something.
As for her Patronus I think it would be something strong and protective - a dog of some sort perhaps. I know Ron's is a dog but there are lots of types of dog.