i feel you. i've been in similar situations, where the other person knew more about what they believe than i did, or they'd say something that i've never heard, so i really didn't want to go back and forth with them. But those situations also make me go back and study more.
Scripture tells us that we should be "...prepared to make a defense," and that it should be done with "...gentleness and respect." (1 Peter 3:15) So if done rightly it shouldn't cause an unnecessary argument.
so to answer your question i'd just say to use that situation as an opportunity to study up on whatever your co-worker was talking about. and maybe the subject will come up again (since you work with them), and then you'd be able to give them an answer.