Testy, I’m so, so sorry.
I can tell you what the tell us as medical students, but I don’t know if it’ll help.
This shouldn’t just be you and her, have a doctor or a nurse with you, please. One-on-one can turn into a confrontation, it should be more of a supportive conversation.
Take your time, make sure there is nothing that will interrupt you, so that you can spend as much (or as little) time as you need to.
Try and do it in a private place, and one that ideally doesn’t have astrong emotional attachment for either of you. From that moment on that place will be the place you told her, so it shouldn’t be somewhere where that will mean cancelling out good memories.
You have to be honest- answer whatever questions she asks truthfully, and if you don’t know, say that, but try and find out or get someone who does know to talk to her.
She may not be in as much denial as you think- she might have picked up on some of the vibes going around and know that something is up. Try and sound out how she thinks you and the team is feeling, and whether you’re worried.
Then just say what you said here. The cancer will return, and when it does, it will be terminal. Do not use “if” and “might” if these are not reasonable terms to use. False hope is worse than no hope.
DO NOT tell her she is losing a fight. The rules may have changed, but it’s still a fight. This is just a different challenge- one to make every day count as much a possible, and to have a good death. If you can, try and find out what a “good death” means to her, and what you can do together to make it happen.
If she’s in denial, that’s where she wants to be, and you’ll just have to work around that. Obviously you both want to be able to accept his, but if she can’t do that at the moment, that’s ok. Just do your best to say the truth until she hears it.
Again, I’m sorry you have to go through this, my thoughts (and prayers if you’d like them) are with you both.