Mel,
Fractures are common. They can do surgery in extreme cases and re-enforce the area with pins/plates. I would contact an Orthopedic Oncologist. They deal with the cancer in the bone. We consulted with this Dr. in NJ (that is where you are isn't it?)here is a link to him:
www.bonecancer.org/index.php?db=content/Bonecancer&tbl=About&id=1
There are different types of leisons Osteoblastic, and Osteolystic. One is more likely to cause fractures. It would be a good idea to find out which your father has.
Thank you for your thoughts and prayers and please know that I am praying for your family as well.
Karen