What is the ruling on a husband and wife praying together if the husband led his wife in prayer while he was sitting, assuming he was unable to stand, then it became clear to him that he was able to stand, and he had prayed in that state for several days?

It is permissible for a sick person to pray while sitting in an obligatory prayer if standing harms them or delays their recovery. It is permissible for a sitting Imam to lead healthy congregants who are also sitting, as indicated by the Hadith (definition) of the Prophet Muhammad, peace and blessings be upon him. If the Imam prays sitting, it is more cautious for the congregant to pray standing, to avoid difference of opinion. As for someone who prays an obligatory prayer sitting while being able to stand and forgets, their prayer is invalid and must be repeated, and likewise for those who prayed behind them.