Dear Prudence is Slate’s advice column. Submit questions here .

Dear Prudence,

I’m in a horrible situation and don’t know how to handle it. I work as a conservator, and I was at a museum to clean a painting. While on my way to the museum’s cafe for lunch, I saw something shocking in one of the galleries.

I caught my mother-in-law “Belinda” and a man who was definitely not my FIL sitting on a bench kissing. I turned and tried to leave, but she saw me. Belinda caught up with me and begged me not to say anything. My FIL has health issues, so I reluctantly agreed, but I feel horrible. My FIL is a kind, wonderful man and doesn’t deserve this. To make matters worse, I can’t even discuss it with my husband because it’s his mother. The guilt is really beginning to eat away at me. What

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