What is the legal ruling on replying to salutation and respond to the sneezer (i.e., by saying to him: Yarhamuk-Allah after he says: Al-hamdu lillah) during the Khuttbah of Jum’ah ?
The answer: this is impermissible because it’s generally considered to be speaking, which is impermissible during the khuttbah. And because greeting in this situation is impermissible in the first place; therefore, replying to it is automatically impermissible.
Also, the sneezer is not allowed to speak out “alhamduillah” during the khhuttbah; thus, replying must not take place.
Majmoo’ fatawa sheikh Muhammad ibn Salih al-Uthaymeen : volume 16/ chapter: Juma’ah prayer.