Q. If someone misses the Khutba, but makes the Jama'ah, is the Jummah prayer still valid or should the person then pray Dhuhr to make up?
A. In the name of Allah the Most Beneficent and Merciful.
The Khutba is an obligation for the Jummah prayer to be valid. However, it is not an obligation for the person attending Jummah to have taken part in the Khutba.
However, it has been stated in the Hadith that the Angels close their attendees register once the Imam starts the Khutba and they listen attentively to the Khutba. It is necessary for a person to listen attentively to the Khutba and if he has not already started the Sunnah prayers, he should not do so if the Khutba has been started. Even though this is the case, if a person attended Jummah prayer and missed the Khutba but joined the congregation then his Jummah prayer will be valid and he needs not pray the Dhuhr prayer as long as all the conditions of Jummah have been met.
Of course Allah knows best.