Oh, Thipu, your brownie story reminded me of something that happened at our house once.
I was hosting a little party for people at my office, where I was one of the few Jews, to celebrate a win in a case. We were all sitting out on the deck, not inside, fortunately. The head of our section and his wife brought a plate of crab dip she had made. [Crab, like all shellfish, is DEFINITELY not kosher.] What to do?! We knew they had no idea that it was a problem, and it was really nice of the wife to make it. We didn't want to embarrass them. So we didn't say anything at all -- fortunately, we were using disposable plates and cups and flatware, so we just discreetly cleared away any serving pieces and such that weren't disposable. And we carefully watched that plate, and anything that that dip ended up on, to make sure it didn't come into the house, especially not the kitchen! If we'd had the party inside, I suppose we wouldn't have been able to do that, but as everything was outside, we were able to pull it off without making them feel bad. I'm glad we could.