Hi and thanks for the question,
one and only one cigarette might not harm. But its from continuing to smoke one cigarette that we might continue smoking. Smoking has very dangerous health risks as we know. Stopping or failing to smoke this one cigarette could be the source and strength of you stopping definitively smoking.
I suggest you be strong and do not take the cigarette. Not taking it should be the secret of you stopping smoking,
Thanks and best regards,