I agree with what Lara said Phyllis but I also wanted to comment on the reason you posted this thread.

I don't want to sound harsh either, but I do think you've answered your own question at the end there.

They need each other right now, probably more than any other time in their relationship and they most definitely do not need others intervening, especially anyone outside of her family or her group of health care workers.

This poor woman is obviously very ill but luckily she has a very strong faith. You said .... "Problem is, her faith won't allow her to have this done..." and I believe that says a lot. If her faith was not strong then she would not let it influence her choice of treatment.

Right now she needs her fiance and she needs her faith as she confronts this terrible condition. If you were to put doubt in her fiance's mind now about her feelings or the way she's coping, don't you think that would distress her, distress him? She doesn't need that now. She needs love and understanding.

Nothing good could possibly come of you telling him what you suggested. If you really want to do something then support them both without letting either of them know your own views on the subject.

Should his fiance pass away from the cancer, Arthur is going to need of all his friends but if you were to tell him what you told us, then I believe you could risk alientating him all together. You might force him to break his friendship with you, rather than cause his fiance any further distress in the final stages of her life.