The cardinality of the friend_of relationship set between two student entities in the student entity set should be:

One to One Relationship

Explanation:

Given that in a computer systems analysis about an entity-relationship model, a one-to-one relationship is a form of cardinality that involves the relationship that exists between two different entities Y and Z, such that one element Y can only be associated with one element Z, or in the other way round.

Hence, the cardinality of the friend_of relationship set between two student entities in the student entity set should be: "One to One Relationship"