I understand your concern. It is difficult to believe that a marriage is meant to last forever, only to discover that people change and that relationships - even marriages - do not necessarily last forever. This is especially the case when a person's culture and religion creates the expectation that a marriage must be happy and ever-lasting at all costs.
The best you can do is not to judge your mother. I understand that you feel loyal towards your father, but in my experience a person has an affair long after the marriage has broken down anyway. Your father also has a 50% share in this marriage, and if their relationship has broken down, he is as responsible for that as your mother is. This may be difficult for you to accept.
If you talk to your mother about this like an adult, you will probably discover that she has lots of confused emotions and that she does not quite know what she wants. Because of religious and cultural constraints, she may feel that she cannot end her marriage, and that she has no option other than to seek love in the way she does.
You are at the beginning of your own adult life. You will discover that life is not as simple as you want to believe, and that people do things that make no sense to you, but that is the way we all live and learn.
Rather than judge your parents, you may want to observe how they both deal with the situation, and then decide what you feel is right for you. And even once you have made that decision, you may change your decision later on when you gain some life experience.
Whatever happens, remember your mother and father both love you and want the best for you. Stay away from blaming either or assuming one of them is right and the other is wrong.
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