I assume that being Catholic she believes that you would go to hell if you committed murder/assisted in a suicide does she really want that for you. If it would be inappropriate or too insensitive to talk like this in your case you could try talking being arrested, going to prison, friends and family disowning you etc etc.
I accompanied a client who I loved and cared for very much to hospital when she broke her hip, she already had several health issues including COPD, tendency to depression and was registered blind. She kept asking for more pain killers - I think she did genuinely need them but then she started begging me to put a pillow over her face. I told her "you know I can't" but I did say I would pray for her suffering to stop. I am sure the only reason she agreed to the operation was because she was high risk for anaesthetic and thought she would not make it through.
I hope you are both able to find some peace very soon.