Is the Friday Prayer valid for someone who only caught the prayer but not the sermon, or must he pray Dhuhr?

Whoever catches one rak'ah of the Jumu'ah prayer has caught it, due to the Prophet's (peace and blessings be upon him) saying: "Whoever catches one rak'ah of the Jumu'ah prayer has caught the prayer." And whoever arrives while the Imam has already risen from the bowing of the second rak'ah, must join him with the intention of praying Dhuhr and complete it as four rak'ahs. Attending the khutbah (sermon) is obligatory, and it is forbidden to engage in anything else during it.