What should he do if he does not understand anything of the Friday khutbah?

Praise be to Allah.

If you listen, even if you do not understand anything, you will be doing something good, in sha Allah. Some of the scholars excused those who do not understand the language of the khutbah and allowed them to occupy themselves with dhikr or reading Qur’aan, subject to the condition that they do not disturb others who are listening to the khutbah. The view which I favour, however, is that you should listen even if you do not understand.