In "Chike's School Days" what shocks Elizabeth so much that she nearly dies, and what does she do to try to correct her son's mistake?

In Chike's School Days, Elizabeth is so upset by her son's announcement of  his impending marriage to an Osu that the 'shock nearly killed her.'

In the old days in Nigeria, Osu were a people set apart. They were viewed as a type of 'living sacrifice' to the gods, and as a consequence, were not allowed to participate in any of the typical ceremonies or rituals of day-to-day living. Osu were only allowed to marry other Osu, and most Osu were shunned by the common populace. Anyone who married an Osu was considered cursed. Achebe tells us that Osu were not allowed to own land, to marry free-born Nigerians, or to take 'any of the titles of his clan.' So, when Amos, Elizabeth's son, announces that he is going to marry an Osu, despite not being an Osu himself, Elizabeth is grieved beyond measure. She begs her son to reconsider, but he does not relent.

The story of Nigeria's Untouchables.

In spite of her rejection of the old Nigerian traditions, Elizabeth visits a traditional diviner who tells her that her son's insanity is a direct result of his conversion to Christianity, the white man's religion. He insists that the only remedy for this disturbing state of affairs is to complete all the necessary, old rituals for the benefit of the ancestors and to sacrifice a goat to appease their anger. Elizabeth does all of this, but her son remains strangely recalcitrant. He marries an Osu girl named Sarah, and the grief actuated by his actions prompts Elizabeth to reject Christianity and to return to her old faith.