Sure, bad stuff can happen. If you trust your best friend to take proper care of your car, then don't worry about it. It's not the end of the world if an accident happens.
IF your friend has an accident - THEN - your insurance rates COULD go up IF he was at fault for the accident.
You COULD get sued - IF your friend causes an accident and the other person is seriously hurt AND there is not enough insurance to cover the injuries and you HAVE something worth suing for. 99.9999999% insurance will cover it all. People have their own insurance for under or uninsured people in most states.
An awful lot needs to go wrong for you to be negatively affected by this. On the flip side, you are doing something nice for your best friend. Seems to me the choice is obvious.
Go on vacation and relax.
So how long would this person be a good friend if they ruined your financial future-just not worth the risk of losing your security or the friendship.

( I knew she didn't have money to pay for any damages.
He smashed up the car - his fault, - in such away that they had to tow & leave the car at a mechanic in Rhinebeck. It was undrivable. So they couldn't drive it back to the city. 
After that incident, my friend kept her yap shut about ever asking to borrow my car. 
. Maybe laws are different in your state but here they very well can garnish wages, have access to funds in retirement accounts, etc. and it DOES happen but on a more simple basis, if there is an accident and a claim is filed the owner of the car is the one that is going to have to deal with jacked up insurance rates, not the driver of the car.