I have heard "good luck" said in the hall, by an elder, from the platform. As with many things, while deemed inappropriate because of its origin, it has become a figure of speech in our modern era. Depending on a JW's personality and rationale, they will be more or less inclined to use it. True, it is frowned upon, and the society has admonished its followers of its pagan origins, but it's one of those things that, if not said often, and in front of brother stringent, it would be shrugged off. Now, if you ask any of them if using "good luck" is acceptable by a Christian, well good luck getting any answer other than no.
I was was once admonished that saying "unfortunate" was bad, as it was a derivative of fortune (as in fortune telling). How can a person using the English language not use the words fortunate or unfortunate (fortunately, unfortunately), at some point? They have become an ingrained part of our language and evolved in their meaning, beyond what the initial root word intended.
...but, you could use that in your argument. Ask if they have a problem using the words fortunate, unfortunate.