a woman is deemed invalid. In this regard, Al-Imam Al-Shafiee'(May Allah have mercy on his soul) said (What means):"If a woman leads males and females in prayer, then the females` prayer is valid while the males` isn't, because Allah made men protectors and maintainers of women for the latter are incapable of being so. Therefore, a woman is never permitted to lead a man in prayer no matter what the case may be." [Al-O'om 1/191]. Kindly refer to [Al-Mawsoa'a Al-Fiqhia 6/204].
However, men being unable to lead the prayer due to an illness or the like isn`t a valid excuse for getting led in prayer by a woman. It is incumbent upon the one who cannot perform obligatory prayer properly to learn how to do so and in the best manner possible. Also, one who prays sitting on a chair is permitted to lead others in prayer and the prayer of both is deemed valid since the Prophet (PBUH) has led his righteous companions in prayer sitting on a chair. And Allah knows best.