Would you tell a vegan that they ate an animal byproduct?

So my coworkers and I always have lunch together. My vegan coworker saw what I was about to eat and asked if she could try some. I had brussel sprouts, tofu, and squash. I let her have some because I had a lot of food. While I was eating, I suddenly remembered that I had used butter to cook the brussel sprouts. I was tempted to tell her right away but I didn't want to embarrass her and myself infront of everyone else at the lunch table. I know she only took two halves of a brussel sprout so I thought maybe it wasn't a big deal. So I didn't end up telling her 😥 what would you have done?