There's difference between sexual (also known as lustful kiss) kiss and friendly kiss (which is often given to friends).
I have watched the video, and it was a demonstration of friendship and appreciation.
There was nothing sexual in it. It lasted less than a second.
It's like when you kiss your friend.
That said...a person can feel uncomfortable and awkward if they are even hugged by friends. Let alone be kissed on the lips for a second or less.
I think he should apologize and tell her he won't do that again.
But resigning is absolutely disproportionate to the seriousness of the act, which is clearly an act of affection. And not of lust.