Welcome to Involuntary Manslaughter anon
Involuntary Manslaughter:
>Homicide that is committed without the intent to kill, ***but with criminal recklessness or negligence***;
>Or, a death that results during the commission of or flight from a misdemeanor or felony
Now, lets look at the legal definition of criminal negligence:
>Criminal negligence refers to conduct in which a person ignores a known or obvious risk, or disregards the life and safety of others
So here you have a guy who took his finance's van-(which was legally hers), leaving her in a hostile environment without any means of shelter, transportation, and if they prove he had her phone, communication. That easily clears the bar of criminal negligence in Florida.
Once you have that, then you just have to make the case that's she's dead as a result of his negligence-(which is a layup argument) to ring him for Involuntary Manslaughter.
Hell, a good DA could even make the case he stole the Van, which would satisfy the charge on the basis of flight from a crime.