The author is in an incredibly difficult position. Discovering a parent's past infidelity is shocking, especially when it contradicts their present image. The author is not obligated to reveal information that could cause immense pain to their father and disrupt their family. Protecting their father's peace of mind is a valid and compassionate choice. The resident may not know the affair happened. Why stir up the past when it can destroy a present good thing?
While the author's concern for their father is understandable, suppressing the truth indefinitely is not a solution. The author is essentially condoning the mother's past actions by concealing them. The father deserves to know the truth about his wife's infidelity, regardless of how long ago it occurred. The author's silence enables a continued deception. Also, it's possible the resident has no idea he was part of an affair, opening up a new can of worms if the mother is confronted.
The situation is complex. While the father has a right to know about his wife's past infidelity, the potential consequences of revealing this information must be carefully considered. The author must weigh the potential for short-term pain against the possibility of long-term resentment and distrust if the truth eventually surfaces. Ultimately, the decision of whether to reveal the truth rests solely with the author, balancing the impact on all parties involved.