What is the ruling on following someone in prayer who prays alone for the obligatory prayer and does not vocalize the takbirat nor the recitation in a loud prayer, and is vocalizing them obligatory?

The majority of jurists hold that it is recommended for the worshiper (ma'mum) and the individual praying alone (munfarid) to whisper all the takbirat. However, the Malikis are of the opinion that the takbirat al-ihram should be recited aloud. As for the Imam, it is sunnah for him to recite the takbirat and the recitation aloud in prayers where loudness is prescribed (salat jahriyyah), while it is sunnah for the worshiper to whisper the recitation. The individual praying alone has the choice between reciting aloud or whispering, according to the Hanafis and Hanbalis, whereas the Malikis and Shafi'is recommend reciting aloud what is typically recited aloud. The prayer is valid even if the Imam does not recite the takbirat and the recitation aloud.