In a Person's life, most important factor in maintaining relationship is 100% transparency. There may be a chance to add up more lies upon one another and finally end up in hearing the truth from a third Party. Here the "Trust" factor breaks. So how can he believe his friend in any other deals? Quite impossible. So whatever it is, be truthful and may be the solution needed is just a "Sorry" from him to continue the relationship. It is easy to break up something, but not easy to reunite it. Once broken cannot be combined.
I would like to narrate a story which broke the relationship to strengthen my point about Truth.
Long time back, when I was in my college, my roommate in College hostel seems to be missing during the study vacation. I got a phone call from her mother asking about her. I called her boy friend and asked If he knows where she is. I am pretty sure, without his knowledge, she won't go anywhere. He started telling lies to me and ended up saying the truth that she's with him. As an Individual, he was one of the best students in College and with this incident, the "Trust" factor breaks and all confidence was lost.
Among all possible behaviors, honesty would be my first concern and the most important consideration in a relationship.