Her weight will have been a factor in the tendon going out. It also pushes her risks up way high for the other one. She needs to lose weight. If she's not going to be getting the exercise she's used to, then even more will you need to restrict her calories. I'm sorry, there's no easy way to deal with that one.
But maybe, if you can work on her and help her lose the weight, she might begin to come good. Give her plenty of massage plus follow the vet's directions. Even if you opt for the surgery, she would have to lose the weight first. So who knows how things will go, as she loses the weight? She might begin to heal indpendnetly, especially if you can try to help her make the changes she needs to.
When I look back at my life with pets, the stand-out moments are the ones where we had to say goodbye. Often I didn't get to say goodbye, the animal was just whisked away as if they never had existed. I was never told what had happened (or some lie was told) and I often wondered, but was afraid to ask. The last time, I was told my dog had run away. He had in fact been hit by a car, I was finally told by my ex-brother in law six months later when he caught me going to the police to report a pedigreed dog probably stolen, plus putting up posters around the town (my parents had forbidden meto, but this was MY dog).
My parents thought they were sparing me the pain of having to face my pet's death. They were wrong. But they thought they were doing the right thing.
Years later, it still stands out in my memory - whatever happens, be open and truthful with your kids and where possibly involve them in the choices that must be made. Give them full information, including the financial situation. Frannkly, planning to do such surgery on an old, overweight dog is risky for so many reasons. Probably foolish for the same reasons. But there are always choices. I'm also with those who say, let the dog help you decide. She is the one who would have to go through any procedure, and it could be just too much to ask.
Marg