If I understand you correctly, your Mom first was told it was not cancer and then when surg was done , it was cancer.
Whipple's are done with "Cure" in mind. Your mom is a great candidate. Ampullary cancer is the door to pancreatic cancer. Look at the anatomy. Yes, the surgery is big, I had it! However, to look forward to a possible life without cancer outweighed the serious surgery.
This cancer is a "sneaky" one, not to be messed with.
Good luck to you and your Mom.