I also agree with the second opinion as a matter of urgency.
Just setting aside the second opinion for a moment though, I think you need to decide what you'd gain from a biopsy. If you would not be planning on doing an amputation, it seems pointless to do a biopsy because it's very painful and then after it's been done, the dog is still in the same situation. If you are planning on an amputation, a biopsy is still pointless (in my opinion) because the results take a good few days (sometimes up to 10 days) to come back and that's a lot of time to loose with something like bone cancer (particularly if it's osteo).
Amputation is a very personal decision and the suitability must be based on the dogs overall health, temperament regarding vet visits, the owners personal situation etc etc etc. I do have an amputee (front right osteo, October 2007) and she did fine with her chemo too. Lots and lots of dogs do fabulously well coping with the surgery and also with the chemo and some of them are lucky enough to have full and active lives for some time afterwards. My Darcy is just over 25 months post diagnosis/amp and there are others who are approaching this stage and past this stage - and there are many many others who lived for a shorter time.
I wish you all the best with your very hard decisions