What is the ruling for a sick person who can pray standing alone but cannot follow the Imam due to the Imam's prolonged standing?

Primary text

It is probable that standing remains obligatory for the sick person praying alone because standing is a stronger pillar (rukn) necessary for the validity of the prayer, whereas congregation is not essential for prayer validity. Alternatively, the person is given a choice between praying alone standing or following the Imam while sitting, because leniency was granted to allow the sick person to abandon standing when the Imam is unable to stand for the sake of congregation; thus, leniency is even more applicable here. This latter view is favored and represents the Madhhab of Al-Shafi'i. This preference is supported by the reduction in reward for a seated prayer compared to a standing one, and the greater reward of congregation over individual prayer (by twenty-seven degrees).