It was an unusual problem, trying to work out how to tell someone that you've worked out their secret, and that is that they are immortal. Or near enough to make no difference.
We'd been travelling together for a couple months by then, and she'd lived through the worst beatings and spells imaginable. She said she could dodge well, but that couldn't be it.
So I decided to wake her up with a sword through the belly to prove my point.
My original problem was thus solved, and then I had a new one of where to dump the body.