He was asked:

“If someone catches one rak‘ah of the Friday prayer, then stands to complete what he has missed, should he recite aloud or not?”

He answered:

Rather, he should recite quietly, not aloud. When a latecomer stands to complete what he has missed, he is praying alone during that portion and is subject to the rulings governing someone praying alone. During the portion he catches with the imam, however, he is subject to the rulings governing a follower. Hence, if he makes an error during the portion he is completing, he performs the prostrations of forgetfulness.

Accordingly, a latecomer recites aloud only where someone praying alone would do so. Scholars who hold that someone praying alone should recite aloud in Maghrib, ‘Ishā’, and Fajr likewise hold that a latecomer should recite aloud when completing the first two rak‘ahs. Those who hold that someone praying alone should not recite aloud likewise hold that a latecomer should not do so.

No one performs the Friday prayer alone, so there can be no question of someone praying alone reciting aloud in it. A latecomer is like someone praying alone and therefore does not recite aloud. Nevertheless, he is counted as having caught the Friday prayer through his participation in it as a follower. What is subordinate need not fulfil the conditions required of what it follows. Thus, the required number of worshippers and similar conditions do not apply to the portion that the latecomer completes.

However, the established Sunnah is that whoever catches one rak‘ah of a prayer has caught that prayer. Thus, he has caught the Friday prayer, just as someone who catches one rak‘ah of ‘Asr before the sun sets,