What is the ruling on one who abandons the two declarations of faith on Friday?

If the preacher omits the two Shahadas (declarations of faith) in the Friday sermon, it is not befitting. The preacher should begin the sermon by praising Allah, extolling Him, bearing witness to His oneness, and to His Messenger's prophethood, then send prayers upon the Prophet (peace and blessings of Allah be upon him). This is the Sunnah of the Messenger of Allah (peace and blessings of Allah be upon him) and his Rightly Guided Caliphs regarding the sermon. However, if the preacher omits the two Shahadas, his sermon is still valid. As for omitting the two Shahadas in an admonition or a religious lesson, there is nothing wrong with that.