Because of the significance of the language to their history together, I want their final farewells to be made in French, so I need some help with some appropriate phrasing for a goodbye in this context, as well as an endearment for a good friend, and an appropriate response for the woman.
Of course "appropriate" is relative when you're talking about very close friends.
Though my cursory looking suggested that "adieu" is not commonly used in conversation, I was thinking it might be appropriate when it really is potentially a "goodbye forever" type of situation. If not, what else might be used?
Edit: Some other phrase that might suit the occasion could work. Perhaps something along the lines of "I'm sorry" or "take care of yourself" or something else to lend her comfort in spite of their parting.