How did they determine she has a torn ACL?
I would either get a second opinion or wait - if she isn't limping I can't imagine that she is in any pain. She would let you know if she was.
Is this a trusted, long time vet or someone new? Sorry for all the questions but I am as protective over my pets as I am my kids! If it came down to it - my pets would go to the vet before I went to the doctor!!
I hope it all turns out good for your doggie . . .