He left ‘Arafaat before Maghrib because he was sick

The most correct opinion is that staying in ‘Arafah until the
sun sets is obligatory, because the Prophet (blessings and peace of Allah be
upon him) did not leave before the sun set, and if it were permissible he
would have left before the sun set, because then it is still daytime and is
easier for people. Moreover, if a person leaves before the sun sets, he has
forsaken the Sunnah of the Prophet (blessings and peace of Allah be upon
him) for the way of the Jaahiliyyah, because the people of Jaahiliyyah are
the ones who leave ‘Arafah before the sun sets. If the one who does that
does it deliberately, his action results in two things:

(i)Sin

(ii)According to the majority of
scholars, he has to offer a sacrifice as ransom (fidyah), to be slaughtered
in Makkah and distributed to the poor.

But if he left ‘Arafah before sunset out of ignorance, he has not sinned but according to the majority of scholars he has to offer a sacrifice, which means slaughtering a sheep in Makkah and distributing it (its meat) to the poor. End quote.