Likely negligent homicide. Maybe involuntary manslaughter. No mens rea.
Stupid’s not a crime. Discharging a gun within city limits might, which is what the charge would be if she’d have hit the vest.
It's been 20+ years, but a manufacturer tested his new vest with a 44 Magnum—that he aimed at his stomach. He said he hated being the guinea pig, even backed up with a couple of telephone directories stuffed underneath the vest!
≡≡8-O