Help, over my head

I am a 28 yo male with a girlfriend (approx 2 year) and the other day I got the heartstopping” im pregnant” phone call. unfortunately that call did not come from my girlfriend but from my … Mistress/friend. I do not want a child and I am not in love with the mistress but even knowing this she plans to keep the child. during our times together the mistress and I agreed that this could not happen, we used birth control and agreed to prevent this at all costs. Now that the situation is real she has changed her views and wans to keep the child. I understand her maternal side and have sympathy for her, and told her the choice is solely hers to make. what should I tell my girlfriend if she decides to keep it and do you think it best for the child to have a mother and a father who do not love each other around or should I just leave them be? I know this is completely fu**ed up but please I seek genuine advice, not a “ thats what you get” lecture I already know the error in my ways