I'd say that the situation is a bad one to be in, and bailing out immediately while saying that the guy can have the car would be best. The only reason a criminal moves you from crime scene 1 to crime scene 2 is to do things to you he couldn't do at crime scene 1.
I also agree with Son Of Vlad Tepes that it is best to avoid the situation altogether by preventive means. If you're waiting for someone, don't park, idle curbside with clear exit route. We talk about buying quality guns in terms of "what is your life worth?" but saving $5 of gas isn't worth the exposure. Keep vehicle in drive, foot on the brake. Windows up, doors locked, pay attention for returning SO.