Is zakat obligatory upon a friend who possesses liquid assets and real estate for trade, and a year has passed over them, but he does not possess sufficient liquidity to pay zakat on them, and he would have to borrow in order to fulfill it?

It is permissible for your friend to delay paying zakat until he finds liquidity, so that he is not forced to take a loan. This is in accordance with what is stated in Al-Kashshaf al-Qina' by Al-Bahuti, concerning the permissibility of delaying zakat if it is difficult to extract it from the principal amount. Also, in the fatwas of Ibn Baz, it is mentioned that a person who is unable to pay zakat because he has no money other than the zakatable asset itself is given an extension until he sells it and then pays zakat for all the past years.