If the boss has already called the police and a report has been generated, there really is no "best case scenario," at least in terms of whether she will be charged or not. If a warrant hasn't already issued, it is very likely to happen in the immediate future.
Hitting the b/f with her fist can be charged as a Class A misdemeanor: Assault - Bodily Injury.
Hitting the b/f with her car can be charged as a 2nd degree felony for: Aggravated Assault - Deadly Weapon. A car is not legally considered a deadly weapon, per se. However, it can be proven up to be a deadly weapon by a jury, if it is used in such a way that it can be considered a deadly weapon.
For leaving the scene she could possibly be charged with the felony offense of Evading Arrest/Detention. There's not enough facts given in your inquiry for me to be able to say how likely the Evading charge would be, but there is the potential for that.
The best thing your friend can do is to hire an attorney and refuse to speak to law enforcement. She needs to not speak about it with any friends. She needs to not text about it. She needs to not post about it on FB or MySpace or any other social media. In a situation as convoluted as you've described, she needs to invoke her right to not incriminate herself, and let her lawyer take the reins from there.
Example: if she speaks to law enforcement and tells them the assault was an accident, she could still be convicted of the offense if the DA's office charged her with committing the offense recklessly. So, if she isn't trained to spot all the issues that can come up when speaking with authorities, the best solution is to not do it.
There may also be some legal defenses available to her. If successfully argued, a valid legal defense might help her get the case(s) dismissed, or get her acquitted at trial. Those may be screwed up too, if she speaks to law enforcement without knowing the potential effect of every word she says.
If the wheels of law enforcement are already turning, she needs to move quickly to be prepared for what is coming.
Answered on Dec 11th, 2013 at 3:14 PM