I am a PhD student who gave a talk in a conference three weeks ago. The conference had this condition that only three names will appear in their "synthetic program" with the speaker's name as the first author. The names of other authors will appear only in the book of abstracts. So when submitting the abstract (4-5 months ago) I first added my name, then my principal supervisor, then my industry partner (who covered all costs for me attending this conference). I believe that I entered my co-supervisor's name as the fourth author. The book of abstracts was published three weeks ago but I did not check whether my co-supervisor's name is appeared in the book because I was certain that I entered his name while submitting the abstract. Unfortunately, one week ago I realized that his name was missing from the book.
About the conference
It was a prestigious conference that was held abroad. However, we did not submit a conference paper. All we submitted was just a one paragraph (7-8 sentences) abstract that included only general aspects of the problem we studied without giving any results.
About my supervisors
I really like my principal supervisor. He is very kind and supportive. Each time we have a meeting I learn something new from him. My co-supervisor is a post-doc in our department and not a faculty member. I chose him as my co-supervisor around one year ago. I ask him to become my co-supervisor after my previous co-supervisor left our university. I mainly considered him as a friend not a co-supervisor and that is the main reason why I asked him to be my co-supervisor (he does not really help with my studies). I am a relatively independent student and I never expected him (or my principal supervisor) to contribute. However, I expected him to know that I will never remove his name from the list of authors intentionally. I am mature enough to know such basic principles and based on our conversations I think he must have realized this after 2-3 years being friends.
How did I realize that his name is missing?
Around 10 days ago right after my flight, my principal supervisor sent an email to me and asked for a meeting. I went to the meeting with my co-supervisor and after I explained my works my principal supervisor told me: "what were you thinking you did not include X's name in the paper?". Then I told them that the conference had that condition and his name should appear in the book of abstracts. Then my principal supervisor told my co-supervisor that "we should have checked the conference website first". My principal supervisor is the head of school and he is very busy. He is very famous and he does not care about his name being appeared in the conference talk. This is true especially because his field of study is different from what I do in my studies. So I am sure that my co-supervisor had told him about this. After the meeting my co-supervisor sent us an email attaching the book of abstracts proving that his name is not in the list of authors. He asked me for a reference to his name. Then I realized that his name was missing and I explained that it could be an error from the conference side. It might also be that I did a mistake when submitting the abstract. Anyways, I apologized for the inconvenience from this error.
This situation taught me a lesson I already knew, the hard way. I already have a number of good journal papers and at least in this respect I know what is right and what is wrong. I have not seen or heard from my supervisors after I sent that email. We may have a meeting in 1-2 weeks.
What do you think I should I tell them in the following meeting?
Is my co-supervisor's way of handling this situation considered professional conduct? I am really sad as I expected him to tell me first. We were friends and I expected him to know I am not that stupid to intentionally remove his name. With regard to this situation, how should I behave from now on?