Fly from bad companions as from the bite of a poisonous snake. If you keep good companions, I can assure you that you will one day rejoice with the Blessed Mother in Heaven; whereas if you keep with those who are bad, you will become bad yourself, and you will be in danger of losing your soul” 

- St. John Bosco 

The first of the three qualities of a real Christian friendship is that it must be - Morally helpful. To put this negatively, it means that companionship is not a true friend if friendship with this person leads to sin, to a troubled conscience, to a lowering or a weakening of faith, to neglect in the practice of one’s religious duties etc. Friendship should have a positive influence on moral good. The appreciation of the worthiness of the friend should inspire one to similar worthiness. It lifts up, it brings both nearer to God, it is a union with Christ. An intimate companionship is bound to influence both parties, and only a good influence is worthy of friendship. There should be mutual help to avoid sin and mutual inspiration to the practice of virtues. I love you, so I want to take you to heaven along with me. This is what a true friendship should tend to. 

The second quality is Agreement; This point may seem oo obvious to the discussion, for we are accustomed to thinking of friendship in terms of common interests. The ideal agreement of friendship implies the ability to work together harmoniously, with wholesome agreement on the big and fundamental things and agreeable compromise on the lesser things. Normally they must be some compromise, some mutual yielding in regard to personal likes and dislikes, in friendship, few people can be intimate over a long period and always have the same desires at the same time or always be naturally pleasing to each other. There must be a compromise, mutual yielding in such small things as how to spend an evening or how to decorate a room, there must be a mutual overlooking of small faults and mutual respect for divergent opinions, BUT THE COMPROMISE HAS TO BE LIMITED TO ACCIDENTALS. It cannot enter the sphere of conscience. It cannot include such fundamental things as a Creed, moral code, method of worship etc. At least for a Catholic, compromise in those things listed above would violate the first rule of friendship (being morally helpful). 

The third quality and best is SELF SACRIFICE, for by this, friendship endures. St Ignatius, speaking on friendship says; Firstly, a true friendship shows itself by deeds rather than words. Secondly, if one friend has good things, he wishes to share them with the other. Friendship is a perpetual and mutual self-giving. This need of self-sacrifice may be summed up in a few words, there must be patience with defects, rejection of suspicions, constancy in service, a real desire, and a genuine effort to understand each other: “whatever you want men to do to you, do the same to them” (Matt. 7,12). In themselves, these occasions of difficulty are small, arising out of the fact that we human beings have many imperfections. But constancy in facing them and cheerfully overcoming oneself in them requires a high quality of love.