Absolutely, you have every right to forbid smoking in your car. Even though they are doing you a favour. You are the one who would have to live with the lingering smell (and it would stink for a long, long time). And don't let them think that smoking with the windows open is a solution. It's not.
Tell them you appreciate the use of their car and will try to return the favour some time, but, by the way, you hope they'll understand that you cannot allow smoking in your car. You can say something along the lines of being "hypersensitive" to it since quitting. (Although I don't actually believe you're hypersensitive; you experience normal sensitivity, just that your senses were dulled to it while you were a smoker yourself). You're not telling them not to smoke; they can smoke before they leave, and when they get to their destination. How much of a hardship is it to hold off during a short car ride?