Describing the facts:
My friend had set a commitment with me (to have lunch together). Although it's kind of rude to dismiss someone that comes to visit you inadvertently, it is ruder to leave someone with whom you had previously set up a meeting, waiting. So:
Interpretation #1: Either my friend simply lied to me, and actually preferred to hang out with that other buddy for some reason, and made up an excuse because the truth would probably make me mad (it could be because he was mad with me for something, or he got late for some reason and there would be no point in even trying to leave home to meet me, or simply would rather spend time with that other friend than with me, simply because he enjoys his/her company better)
Or maybe he just got lazy to leave home and changed his mind about meeting me for lunch and no one came to his home, in the first place.
Interpretation #2: The truthful option: Someone did come over to his place by surprise and he didn't feel confortable enough to dismiss the person under the excuse of another commitment. Maybe because he's closest to me, he thought I would understand the situation better than the person that droped by at his place.
In face of these two interpretations, I believe the best way to ask for a clarification, would be through assuming the truthful option was the case. First start saying it's ok, that I understand and didn't mind being left waiting, at least not in an irreparable way. Then telling him that he can and should feel confortable to tell me when something unexpected like this comes up and he has to postpone the commitment he had settled with me.
I believe just by telling him this, he would feel more confortable to deeply clarify what actually happened or, if it was the case, to refute the lie and tell the actual truth, or at least start demonstrating some nervous behavior and embarrass himself in trying to sustain the lie.
Either way, this is definitely always the best approach in order to strengthen a friendship: Trying to always tell the truth and express your real feelings about a certain situation.
