Parts of Framingham were/are not exactly Mayberry. On the other hand, we don't know that Simone didn't make it to Hyannis and someone picking her up in Framingham could be passing through from anywhere.
It was the 70s so hitchhiking was more common, and though still not a great idea, in the big picture far more people survived it than didn't. It just happens that it was more prevalent in certain areas and young people going to the Cape and Islands in the summer months would be a high number.
As for mentioning the beige sedan, I can see it being something innocuous. The roommate on the phone asks who she got a ride from, "is he cute, does he drive a Corvette?!" or even giving her the description so friend can alert a parking lot attendant if they lived in a complex with assigned parking, etc.
Darker scenario is the original driver she was fighting with is still in the vicinity, circles back to 'apologize' and she decides to go back with him.