DENIAL will do exactly this.
When told they have cancer, a patient goes through various stages.. the five commonest are anger, denial, depression, bargaining, and acceptance.
patients in DENIAL often do not admit to others that they have a secret irrational belief that they do not have cancer, because they know there will be a fight (since there is so much evidence that they do have cancer - scans and tests clearly show it).
other reasons include fear of the unknown - chemo and radiation can sound much worse than they actually are to someone that has only heard about horror stories through the newsmedia. if this is the case, ask her oncologist if it would be ok for her to visit his infusion room and sit and talk to any of the patients there that day that are willing to share their chemo/radiation experiences with her. this will do a lot to help her understand that chemo/radiation is not easy, but it is no where near the horror she imagines in her mind it to be.
Let me know if any of this helps,
-Amnia