Ideally peanut butter should be stored in a cool place to prevent it from melting.
However if you do leave peanut butter in a hot car nothing bad will happen to it.
I keep a couple jars of peanut butter in my car to snack on and make peanut butter sandwiches when I go to work and have left jars of peanut butter out in hot car temperatures a lot before.
The peanut butter became melted some which was actually a good thing because it made the peanut butter easier to scoop out and spread onto the bread.
But the peanut butter is safe to eat and as long as it doesn't smell bad or have mold in it then the peanut butter is okay to eat even when left out in a hot car.