I am not sure what you're asking. I would like to understand better exactly what we could clarify for you. Does your question relate more to aikido philosophy? Moral obligation? Or, to personal values?
On a general level, Aikido philosophy is shaped by unique principles defined by each individual. It is much, much more than pigeonholing this concept into one block sum; "Trying to help other when they try to do something they shouldn't".
In Aikido's most blatant form violence begets violence. Simply, one violent act reflects and is returned, or the "natural tendency of resistance or fighting back". Perhaps, this is a reflection of society's culture or our culture as humans is fascinating, but a whole other conversation.
The notion violence begets violence, is reproduced in many of the world religions and cultures; martial arts being only one of them. Specifically in Aikido, this concept is reflected on set of values in everyday training: teaching the body to unwind, not to resist when confronted, and to blend or "absorb" (for lack of a better word) conflicts, on and off the mat. Refining the concepts even more, the simplest Aikido roots are based on the aforementioned underlying philosophies. Ex: The circular movements that connect the universe with ourselves, the principles of ma-ai (the relationship between the attacker and the defender), etc.
Trying to answer your father's question more openly; "Isn't that irresponsible? I think you have a moral obligation..."
The question posed can only be answered based on a personal level, probing one's central values, the values that define who you are as an individual. These principles, turn into moral/ethical questions when spliced into a situation, ex; "What's more important his life or mine?" These questions can be influenced/developed by outside sources, such as aikido. So, on that note, the original inquiry offered asks you to paint your own moral canvas, including the teachings grasped by your personal journey, "way", which includes Aikido.
Maybe I am misunderstanding the context in which the viewpoint is presented, but I do not understand how the original question posed, conflicts with any of the teaching in Aikido, in fact quite the opposite. Speaking from my mind, one should take into context each moment/situation a human being is presented with. Personally however, I strive to feel mutual compassion for every human being. This quality, nurtured by my interests in eastern culture specifically Buddhism, reflects and strengthens who I am through Aikido. Aikido is merely a facet for the expression of my inner person, as corny as that sounds. To often, especially in religion, a prepared belief structure often defines who a person is, not the other way around.
Presented with, the situation where an innocent victim was being attacked I would forgo my present obligations and do everything in my power to help that individual. That is not to say I would run blindly into battle, or try and maim every thug on the street. It is to say I wish no harm to anyone; attacker, attacked, human being.
that took along time to write, I hope someone reads it... LOL. More importantly I hope it makes sense.