Q. 1. When is one allowed to read other people's private material such as a diary or personal letters?
A. 1. As a general rule, one never has a right to read other people's diary or personal letters without the consent of that person.
The same applies to matters related to someone's medication. No one has a right to read what it says on a pill container, be it the name of the Doctor or the type of medication that the person is taking. Such is private information.
There are rare instances where reading a part of someone's diary (the last few pages) is justified if the person's well being is at stake. If someone has been missing for a reasonable period of time and no one knows where the person is at or with who, it then becomes an obligation to consider the possibility that maybe something has been written in the last few pages of their diary. This does not give anyone the right to read the entire diary. Such a behaviour is considered as snooping.
In summary, as a Christian, you have an obligation to respect the privacy of others. It is improper to snoop or to pry into the personal affairs of others by asking private questions.