The probability of either you or your friend winning the car is 1/3 + 1/3 = 2/3 As opposed to that, the probability or nether you nor your friend winning the car is 1 - 2/3 = 1/3. So even after your friend leaves the show, the probability of you winning the car with your first choice is still 2/3. If you swap the door, the probability of you winning drops down to 1/3. Therefore you should stick to the door you picked earlier.
