When did she tear it?
Is she putting any weight on the leg?
How is her health otherwise?
Who told you that you need to do the surgery now?
Why now after a year?
I understand now.
It will make her quality of life better. Right now that joint, the tibia and the femur, is unstable. Every time she moves, the ends of the bones move across one another. Each time this happens the cartilage (covering of the bone) is worn away.
Then, the surface of the bones become rough. Two rough surfaces moving against each other is painful. The roughening and wear of cartilage will get worse unless the joint is stabilized.
The surgery will stabilize the joint.
I would have a complete blood cbc and chemistry run to make sure her internal organs are functioning the way they should. I would also have chest x-rays taken to look at the status of her heart and lungs.