I would get a 2nd or 3rd or even 4th opinion one of which should be a
Vet school (<--click here for a list) before they amputate her foot (When our dog was diagnosed with cancer in the fall of 2006 we took him to 4 different veterinary practices hoping have an option for treatment/cure made available, unfortunately due to the location they all agreed on one course of action).
I would also suggest calling someone who has a lot of knowledge of mastiff or giant breeds.... the people at
gentlegiantsrescue could possibly help you out with suggestions on how to deal with a large breed animal with that type of wound. I know they also sell a specialized food for large
breed dogs and they have had great success with extending the lifespan of Great Danes and Mastiffs of the dogs who are fed that formula (I think they had a 20+ year old Great Dane), feeding a proper diet could speed up the healing process, as well as extend her life expectancy
I hope her foot heals properly and that they don't have to amputate.
__________________
~Ignorance can be cured with education~
Mom to:
A little girl (6) and a little boy (4), and 2 dogs

Leo and

Gunner
Proud Coastie Sister!! 
My brother is in Virginia at FT school !!
RIP Ed

October 2000-November 2007