Hello Rachael...I'm sorry your 'sweetie' has this cancer and is soon to undergo amputation.
I don't know alot about osteosarcoma beside the fact that a sarcoma oncologist/surgeon (specialist in this cancer) is what you want for her. Helps to have an oncolgical med team that is very familiar with 'sarcomas and has a long track record to prove it (with high success rate a big + !)
Certainly there are some 'sarcoma warriors here who can help you out with their experiences...I hope they post this week.
Wishing you all the best for you and 'sweetie'...God bless!