Both my parents died of lung cancer, and both were travelers. From what I remember, both felt well enough to travel BEFORE treatment started. And their drs did not discourage it, but did warn each that they might not feel like doing much. Stamina was a bit low, but travel itself was not a problem. As with any lung disorder, they will likely be limited in how much walking or touring they feel like doing once they arrive....but every case is different. And it is a wise idea to have an oncologist, or GP, you can contact in Paris, if need be.
If your family member is undergoing chemo, it's a totally different matter. Once treatment starts, it will be difficult to find a window during which they feel like traveling, or blood counts permit.
Once they complete treatment, it may take a while for energy to be restored to the point where they feel like embarking on a long trip, be it in a car, or on a plane.