In Things Fall Apart, what is a Christian practice that was good for their people, but seen as an abomination by Ibo?
One example of a healthy or "good" practice of the Christians was their willingness to go against traditional Ibo culture when it came to traditions that appeared to be cruel or harmful. They would rescue the twin babies from the forbidden forest rather than allow them to starve and die there. This is particularly difficult because the practice seems to be so completely foreign and cruel, yet was accepted among the Ibo people.
You might also make the case that the Christians are trying to create a problem solving culture based on discussion and diplomacy rather than one based almost entirely on conflict and violence. That could certainly be considered something that is "good" for people, if you are trying to make that argument.
One Christian practice that was am abomination to the Ibo culture is the acceptance of everyone. For example, the Ibo people would place twins in the evil forest believing that they were evil. The Christians thought it was immoral, so they would rescue any twins that were left there. Another example is that there were people known as outcasts in the Ibo culture, and those people were basically not accepted in society. The Christians though disagreed with the Ibo and accepts the outcast inside the church as long as they shaved off their hair (because their long hair represents them as outcasts, and they believed that by shaving it they would die).