I recently had a similar experience. My sister - who has a BS in Biology and is now a veterinarian - has been born-again for a while, as I knew, but I hadn't really spoken to her about it. Several months ago, she decided to break the ice on the topic, by sending me Hugh Ross's book
The Genesis Question as a Christmas present. It's an Old Earth Creationism book, filled with many of the same old creationist canards that we all know and love, minus the young-earth stuff. Hugh Ross is essentially a Day=Age Concordist.
My heart sank a bit. I didn't realize that she was a creationist - in fact, with our background (both parents were PhD biologists, mother went on to become an Episcopal priest - and still accepts evolution) I figured she probably was a theistic evolutionist. Meanwhile, she knew that I am agnostic, but had no idea that I'd had a great interest in the evo-creo debate (and am firmly in the evo camp) for quite a while. With the book, she sent a very nice letter explaining that she wanted to "come out" about her faith and was concerned about my reaction.
I emailed her and assured her that I completely accepted her beliefs and continued to warmly embrace and welcome her. Then I told her about my interests. I told her that I'd be happy to discuss the book, but that I was likely to be very critical of it (in fact I'd already filled the margins with rebuttals). I told her that if we talked about it, I wouldn't get angry, but it would end up as a debate, and I didn't know if she wanted that.
She said that she didn't want to debate, so I dropped it, and so did she. We've had good correspondence since, and she's actually going to be visiting with her family this week. I'll be interested to see if they bring the subject up, although I doubt it. We'll be too busy talking about our kids, I expect.
So, I guess that's all to say that I agree with what NosyNed said - sometimes it's best not to talk about it. But on the other hand, if my sister and/or her husband were to give me a scene like the one you experienced, I would absolutely respond, as calmly and unemotionally as possible. And I would suggest that we continue the discussion over email. As we all know, verbal debate is not the best format for this discussion.